Playing the piano or guitar, even at a semi-beginner level, can be a lot of fun. You won't make money off of it, but you can show off to friends and family members. It's like playing tennis or basketball: you need a high level of skill to compete seriously, but you don't need to be even as good as the local high school team to have fun with your friends.

Incidentally, I think it might be easier to achieve minimal competence on a guitar than on many other instruments - you can perform a lot of songs just by learning a few chords and singing the melody.