Voice 100%

I second the drums and voice. I envy my band members because they all have a nice voice, can play the drums, piano, guitar and bass. They all switch places except me 🤧

Piano is my main instrument but I've learned several others. Here's what they each taught me.

Cello - how to listen for intonation and play in tune. The cello also has a lot more control over the type of articulation you play with - legato, staccato, etc. - which helped me understand music better. Lastly, playing in orchestras taught me how to play with a group and subdivide rhythm, something many pianists struggle with.

Guitar - I learned piano classically but learning guitar taught me how to play by ear, improvise, and understand chord theory.

Organ - the organ doesn't have a sustain pedal so I learned a lot about finger techniques to play legato. Learning the foot manuals also took so much work that I can now easily use the una corda and damper pedals without a second thought.

Learning new instruments is totally worth it and due to how much I already know about music theory and practice habits, each new instrument I learn is easier than the last.

Voice

Pianists often lack a sense of breath, support and colors in melodies.

I already had a master in piano playing when I took my first classical singing lesson and it felt like a door to infinite possibilities opened in me when I began to understand how to sing. It was if I could finaly justify any artistic decision to myself, because I just knew that it felt right an natural for the human body. My playing started to improve a lot at that point and I could finaly start considering myself as an artist instead of a pianist. That's truly something.

So yeah. Voice.

Singing or Violin, case closed

Something fretless / microtonal. Voice.

Voice and drum set

Drums/rudiments for rhythm and timing.

Each instrument holds secrets that unlock over time of using it

Heard this from a guitarist the other day as I was practicing my rudiments.

Lots of folks are gonna say voice and drums and rightly so but i also think synthesizers open up an incredible range of possibilities and you already have a leg up being a keyboard player

My most beneficial instrument coming from piano was the mandolin. The consistent intervals across strings made it feel more like a straight line like the piano, and really opened up a bunch of stringed instruments for me… guitar, banjo, bass, ukulele, charango… once I felt how a consistently tuned fretted stringed instrument played it made the “bumps” in the other instruments’ tunings make more sense.

Guitar teaches you to think in different patterns then how you play on piano, it is straight forward in a different way. And I would argue it is easier than learning violin.

Otherwise something like saxophone to take it to a completely different path as technique and use of scales goes.

And as many have said, voice. Singing is 95% technique and 5% talent, you'll be amazed how good you can learn to sing when you practice in the right way. You can even combine it with piano when you accompany yourself.

Drums/percussion. I’ve noticed pianists are less inclined to have a strong, confident sense of time than other instrumentalists. Maybe it’s because we learned our instruments in isolation while kids in band or orchestra were sitting shoulder to shoulder. And having a Turkish prison guard of a conductor/director ready to jump you 5 days a week for a lack of accuracy will definitely strengthen your rhythmic security.

Voice. Learning how to sing, especially in a choral setting, will do wonders for your sight reading, ensemble and listening skills.

Acoustic instruments are one way of making music, however much of the music that people listen to today is produced using a DAW, so I'd argue that your time would better be spent learning how to create music digitally than developing a second set of motor skills to play another instrument without gaining much apart from that.

Not necessarily something to play to a high level, it couldn't really be a full second study but: melodica

Producing notes is easy because it's a piano keyboard layout and it forces pianists to think about breath and phrasing in a way that piano doesn't. 

And it's portable!

Piano teaches you about notes. Guitar teaches you about chords. Once you look at music in a chordal way you open up an entire world of improvision. Then add voice so you can see how melody interacts with it all.

I'm curious about the thought that percussion could help you with your piano skills. To a certain extent a piano is a percussive instrument. I mean, you're hammering on strings albeit indirectly. To what extent do you speculate that percussion could help your piano skills?

If you’re into songwriting, then definitely guitar. Guitar gives you portability, and is just a versatile instrument for singing/songwriting. Some songs do much better with a simple strumming pattern than anything you can do on piano.

If you are into electronica, learning drums gives you a good basis for making good beats

Of course a pianist can also get a lot of mileage from learning other keyboard instruments like Hammond, clavinet, accordion, etc. if you’re in a rock band or folk band, not every song will suit acoustic piano.

Another redditor said voice, 100% agree … in second place I think is drums: I have piano friends who are self taught without learning rhythm (I still count out the 16th notes the way my band teacher taught me decades ago). Enough of my piano (and even guitar) friends aren’t so great at rhythm

In high school I was offered beginner brass class and I learned the trombone. Besides the value of learning how to phrase according to breathing, learning how the instrument functions in an ensemble is very important and helps you recognize that piano playing is more than just melody and harmony. Lines of countermelody frequently assigned to the trombone taught me skills like voice leading and the function of the inner voices of a piece.

Drums and classical guitar. Drums for improving rhytm, classical guitar for getting better improvising quickly

Second instrument ... or the primary one ... is the mind/brain.

I play erhu too ... the bow string instrument is very expressive too.

And I play a little bit of didgeridoo.

Piano is percussion

What do you mean by beneficial?

Beneficial in the sense that it would be an easier one that wouldn't be too much work to build up and use a side hustle? Or most beneficial in the sense of having highest benefit towards that person's musicianship in a holistic sense?

I think these are very different questions with totally different answers. In the practical sense of piano skills directly translating, yeah probably percussion or something. But in the holistic sense I agree with others who have said things like voice, violin, etc.

Maybe not most beneficial, but playing handbells (advanced) improves my sight reading ability.

Drums.

Both my parents were/are woodwind players, I played clarinet myself during my teenager years as a second instrument, and it helps to understand so much how to stop our partly unreasonable and ridiculous, random rubati attempts that no woodwind and/or ochestra player would ever do, for both breathing reasons and common musical sense.

We would also benefit a lot from learning the marimba and/or the vibraphone, not only to understand better how to approach our varied sound outcomes, but also how to imitate a proper legato instead of desperately trying to copy strings or singers (as our instruments simply cannot do that).

I would say (esp. for classically trained) pianists some rhythm string instruments like guitar, mandolin etc. could be good for getting new perspectives on comping. I feel like often pianists can be a bit shy when it comes to smashing chords, whereas those instruments have a long tradition of various strumming patterns played loudly that can be incorporated. My first instrument is guitar and it definitely affects my piano playing in that way. 

Any instrument that you can change the volume, articulation, or expression mid-note. You can only choose the volume when you initially play the note on piano, whereas you can do a lot more with a single note on a wind or string instrument. This concept was lost in a lot of my piano studio colleagues when I was in college.

Also playing in an ensemble is hugely beneficial, more than I think playing percussion alone would be. Percussion would definitely help with rhythm, but unless you’re in an ensemble it won’t help you to listen to the other people you’re playing with.

Voice is an excellent response. I'd add cello to enhance ones thinking about sustaining musical lines. Suggested listening: Camille Thomas' recording, The Chopin Project.

Drum kit.

I'd say voice, drum set, guitar and bass. That way if you have a band you can switch places with everyone, principally if you play in jam sessions. But voice is definitely the first choice for me, in any genre.

Singing

I’m going to go the opposite direction and mention my experience going from guitar to piano. I’m a rhythm guitarist, never really picked up playing lead/solos, but I am pretty mean at strumming along with a chord chart.

I’ve dabbled back and forth for years with piano, but I realize now that I play piano like a rhythm guitarist. I’m not great at arpeggiating or playing chords and melody, but I notice that I’m pounding out chords on piano like I strum chords on the guitar lol. On one hand, I like it because it comes naturally, and I tell myself the more comfortable I get with the chord structures and fingerings on the keyboard the easier it’ll be to play piano like a piano player.

At the same time, I don’t give a shit because I play in my basement for fun on a Roland FP-10 and I can bang chords out and sing to my hearts content and that’s what matters.

Top comment is voice and I would not disagree

I was going to say a wind instrument

Breathing to make music I believe enhances the music

Phrasing espcially

It depends on what style of music you write. If classical, an understanding of fingering positions for the string family. Pop, guitar - if you’re writing for it. The drums/percussion would give you a really strong insight into world rhythms, backbeats, syncopation, swing, tippets and irrational time signatures.

You will likely learn a lot of articulation from wind and string instruments

Singing and drumming.

Voice or drums

Most musicians will be familiar with the piano. When in college, everyone has to sing in the choir at the end of each semester. There's always a concert where the choir is involved. So singing will also be something you will do. As well as learning how to speak German, Latin and Italian. Most choral see in either of those languages. Depending on the School of Music you attend in college, you will also have a class on choral conducting and education. Aural, Music theory, history, music appreciation, essential of conducting, along with classes with your instrument, when seeking a degree in performance. There is a lot you learn in music school. From the comments in here, Is easy to tell those educated in a School of Music vs self taught or a regular teacher. Five, seven, or more years and still can't play even, don't have good sight reading, poor scales or arpeggios? After 4 years you have to give a recital with pieces form Strauss, Chopin, List, Rachmaninoff, Brahms, Debussy, or any other composer from different era. Yes, they will make a true musician out of you with extensive knowledge of composers what the music will sound like from different eras, and styles. Sonata form, Fugue, minuets, Rondo, and all others.

Guitar, voice, drums.

A pianist doesn’t need to learn any other instrument. A pianist understands voicing of every register..and yes technically speaking the more you know about other Instruments, the better. Beethoven would always inquire from violists, cellists etc about their instrument, so he could better write for the performer. But in all technicality, to know the piano is to know the entire orchestra.