I started playing bridge during my PhD, at 25. it's a very interesting strategy game, once you get over the "game for old people" stereotype. If you can find a few friends to learn and practice with, I definitely recommend it.
There is also a very nice web-based community (so you can play it even without friends and without leaving your mom's basement); most active players play online on bridgebase.com, including the pros -- on that website you can easily find world champions playing a hand or two just to chill out. It's free (as in beer), unlike the face-to-face tournament which are sorta expensive.