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[maplecleff1215]

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[dogperson]

As a child, I simultaneously studied and played several different instruments in addition to the piano.   As a returning adult to the study of the piano, I have thought several times about pursuing learning the cello again in addition to the piano.

I have stopped myself from this emotional response, because it takes a great deal of time, practice  and unique skills to play any instrument well.   For me, learning piano is a lifetime commitment and I cannot split that time and effort with another instrument.   Others manage to do this, but I don't believe it would work for me.  If I started the cello again, I would not be content to play it poorly.

 Maybe someday, if I can quit my day job....

[hardy_practice]

You already have a second instrument that you're good at - join a choir.

[mjames]

Play guitar, it's easy, cheap, and girls seem to like it.

[keypeg]

Play guitar, it's easy, cheap, and girls seem to like it.

Yes, when I was still a girl I learned classical guitar, and absolutely loved it.

[dcstudio]

Having a secondary instrument is mandatory to receive a degree in music from any university.  If you are not a pianist then piano is automatically your secondary instrument as you will be required to pass a barrier exam on the piano to move on to the higher level music courses. If you are a pianist, or you can already pass the piano barrier then you must choose a secondary. 

Voice is the recommendation

You don't have to kill yourself on a secondary instrument and studying voice is so helpful..

[maplecleff1215]

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[indianajo]

Other instruments have much more opportunities to play in groups than piano. 
In my part of the country, entire festivals are held, and party "barns" built for people to get together and play guitar, banjo, mandolin, fiddle, and string bass.  As one can play them standing up, a lot of informal playing happens in parking lots and campgrounds of "festivals".  At lot of friendships are made this way, leading to home visits.
At one time people got together in quartets to play violin, viola, cello. There is a lot of literature out there for that instrument group. 
My band director induced me to drop piano at 16 and concentrate on the bassoon the school loaded me.  There were amazing results including being invited to apply to 150 college music programs, being invited to play the Rose Bowl etc.  But the opportunities to play that instrument as a volunteer are so limited I never bought one.  All three amateur groups around here those parts are held by two ugly old men. 
Piano seems necessarily a solo activity. I've found a couple of churches full of the very old who appreciate the practice of singing to a piano, but modern churches are looking for guitar bass and drum players. Other than that there are nursing homes that have a captive audience.    Transporting the piano to a park or festival is a non-starter, and those electric imitations are reviled by me and not appreciated at all by the "country" music fans in my part of the country. 

[maplecleff1215]

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[Bob]

Strings can help your ear for tuning better. 

Percussion in general or rhythm section percussion can help with being exact on the beat, along with being able to keep a steady beat, knowing different tempos, etc.

[indianajo]

I'd also like to play an instrument that I can play with others and transport more easily. The result of the bad wrist is that I can't have my hand flipped over too long or strain it in an awkward position. This rules out instruments such as violin and guitar (I've experienced this with guitar).

"Left handed" guitar may still be a cheap and available option.  There are instruction books for left handed guitar. There are instruments sold for that.   The hand that does the picking/strumming has the wrist fairly straight. It is the fretting hand that has to twist far. So reversing the hands may solve your problem.   

[hardy_practice]

I can't sing. :/

Of course you can.

[Bob]

Obviously it's better to learn multiple instruments for a number of reasons.

I'd question that statement a bit.  I've heard the opposite.  For teaching, yes, learn them all, but that's only at a beginner or middle school level.  But taking them seriously?  If you do more than one, you've split your time, and both suffer.  If you focus all your energy into one instrument, you make more progress in that one instrument.  And then you're competing with everyone else.  And there are plenty focusing solely on one instrument.