First choice would be piano, I love the physicality of it. Second choice would be a massive drum kit. Again the physicality, and the amazing coordination.
New guy to the SDMB; pardon my party-crashing.
Played clarinet, baritone horn/euphonium/whateveryacallit, and bassoon in high school - bassoon on into college.
What I would really like to play is an orchestra - as a respected and feared conductor.
Failing that - hammered dulcimer, please?
Try your local New Agey type store. Seriously.
I would love to master the violin, myself. I took lessons when I was a kid until my mid-teens, but never really had the determination to get any better at it. I still have my violin, though.
As a musician from a family of musicians, I can already play about a dozen instruments, and some of them pretty darn well, if I do say so myself. 
But I have always wished I could play classical Andres Segovia-type guitar with his level of skill and artistry.
Flute.
I played through high school and into college, giving it up because of time. MayGirl plays my flute now, and she is so much better than I was at that age. Through her, I’ve learned the range that a flute has. I want to take it up again, but that would necessitate buying another flute.
The bassoon is my instrument as well! Got my first professional gig at 18 in 1988 and never looked back.
I’ve conducted as well, with enthusiastic response from the ensembles, but I was never tempted to make it my profession. My other musical professions are composing and teaching.
It has been said that a gentleman is someone who can play the banjo - but doesn’t.
I have to go along with Czarcasm on this one. Pipe organ, the bigger the better. I look forward to mastering the technique of using my feet to perform arpeggios on the pedals - the so-called “Tootsie Roll”.
That is wonderful, Knorf; I knew I was never good enough to go pro (though I did, in fact, self-teach myself bassoon). I still am inordinately proud, for no reason at all, that I played both single reed, double reed, and brass all at about the same time.