If you could play any other instrument, what would it be and why?

Singing or violin, I often miss another instrument. I often miss to have the opportunity to follow one melodi and to be more social in music.I would also like a future “Tonebase Voice”

Cello or Clarinet! Both very soothing sounds. I've taken cello lessons and would love to start again.

Well, the harpsichord, but I'm not sure you are lumping keyboard instruments together.  (Love those smaller keys!  Maybe like Chopin's Pleyel.)  To me Les Barricades Mysterieuses sounds mushy on a piano.  But if I were choosing a completely different instrument, the Renaissance lute would win hands down.  

UPDATE:  When Magdalena Stern-Baczewska plays Les Barricades on the piano, it does not sound mushy.  

I think I would choose voice. I grew up in a family of wonderful singers, and while my voice is not great, I am moved by beautiful a cappella singing more often than by any other "instrument." Indeed, I have had trouble connecting to the piano. The instruments I played previously (trombone and double bass) were quite physical in what you have to do to play them. I miss that, and sometimes when I play the piano it somehow feels like I'm not really 'making music.' Just a few weeks ago, though, I finally found a musical/physical connection to the piano, and it was through voice. Learning a new piece with my teacher, he asked that I sing one line in solfege while playing the other line, Practicing this, I suddenly found myself "feeling" the music. So, I'd really love to learn how to sing better both because singers impress me with their abilities and so that I can continue to feel a connection to the music I am playing.

Violin , Viola , Cello or Harp

Organ.

(Would love to see Tonebase Organ!)

Have to say guitar. I love the tonal flexibility, no blowing required (been there, done that with the trombone and trumpet) and the fact that you can take it with you just about anywhere - unlike my 5 foot Chickering baby grand . 

Voice and cello, sitar, saxophone 

I originally said "guitar" (I played it in the past) but now I would REALLY like to learn to play organ!

I have two instruments I would like to learn.  I played the flute in high school though I never had a teacher.  Now I would like to actually have a teacher.

The other instrument I would like to learn is violin.  I had a semester of violin in college, but would really like to expand my ability to play the instrument.  Though I don't currently have either instrument, it's a desire I have, and will pursue it when I have the opportunity.

I would say voice and guitar. Even though my main focus is classical piano, I would only do pop for voice and guitar, thats just what happens to interest me. Although after listening to Jakub Józef Orliński, maybe I'm also interested in classical voice a little bit who knows.

I play a few instruments (piano, cello, guitar, started singing in the past year), and I wish I could learn more, but at some point I've had to cut myself off from picking up new ones so I can get things done with the ones I have.

That said, if time and resources were infinite, I would love to learn both the oboe and english horn (which as I understand are fairly similar, so most oboists can easily pick up the other). I just love the expressive, lyrical, somber quality of those instruments.

All the instruments of an orchestra? :-) 

Realistically speaking, though, perhaps I could only try to play the organ in this life time.

I would love to play drums! Very passionate instrument, It would be very cool to play it, with a band

how i love to be able to play violin or cello? but there are so much to learn in piano that I just can't seem to find the time to learn another instrument. maybe one day. 

I would like to learn singing too!

But for the instrument, it would be harpsichord. And organ - that’d really be an achievment…

I have tried a couple of times to learn classical guitar and I still love the music but piano seems a more approachable instrument and both are challenging!

I would love to play the Uillian Pipes. I love the music of the Chieftains and other Gaelic groups. The pipes are crucial instruments for the expression of Gaelic music. They are much softer than the Scottish bagpipes, which give me tinnitus, and not an instrument of war. The sound is ancient, and can quickly change from a rhythmic jig to a mournful farewell. It can be played without ear damage in small areas, such as the gatherings of ‘kitchen parties’ in Nova Scotia or the Ceilidh get-together in Ireland and Celtic areas of Brittany, etc. 

Nothing beats piano!