I've been playing guitar for 12 years, and even though I don't play as much as I should, it's still one of my favorite things to do. (Here's a pic of my guitar collection.) Surprisingly, quite a few baseball players also play guitar. The only card-related evidence I've seen of this is the 1991 Stadium Club card of Eric Show (photo courtesy of The Baseball Card Blog).
(Guitar geek note: I'm 99% sure that's a Gibson ES-175. It's a beautiful thing, isn't it?)
Other players that are known for playing guitar are Barry Zito, Bronson Arroyo, Ben Broussard, and Bernie Williams. (If I'm forgetting anyone, let me know.)