When I was a kid Mom tried me on trumpet and guitar. I think I might remember one fingering on the trumpet, but every now and then if one is around I check to see if I can still blow a steady tone...I can. So, *in theory* it would be a matter of brushing up on the fingering, and building stamina. Guitar didn't take, 'cause I wasn't interested in learning what was being taught. I went without playing anything through most of school. In University I picked up guitar again, and made real headway, but I had a mentor through most of that time. Kept up with guitar, without really doing anything constructive for quite a few years, then started the band I'm still in.
Eventually the band got tired of me firing the bass player, so I moved to bass (that worked, I immediately stopped firing bass players...). Part of our longevity I have down to periodic hiatuses, which I've often used to play around with some other instrument, so I've picked up a smattering of tin whistle, mandolin, and banjo at various times, but never seriously, tin whistle was probably the one I was closest to any proficiency in, and that wasn't much, definitely not stage-ready anyways.
This hiatus (we're on one now) for some reason, I've been playing guitar more, which is really strange, usually I go for something different. Had a meeting with the boys last week, hiatus will be extended more than usual, largely for the singer's benefit, but the rest of the band will still be getting together to write and one suggestion the guitar player had was we bone up a bunch of cover material with him and I switching instruments. He borrowed a bass of mine when we went on hiatus and I gather he's been enjoying playing it, and without the singer, we'll have to handle the singing amongst the three of us. All of this is going to help when we come off hiatus, particularly since I'm much more prolific a writer when I'm playing guitar regularly, go figure...