Beach or sand volleyball is another great sport for going shirtless. In fact, it's pretty much expected for shirts to come off in this sport. Afterall, swim trunks or board shorts are standard attire for men at the beach anyway. It's a lot of fun, great exercise, and a good chance to get some sun.
Even an indoor volleyball game could go shirtless, though.
So, get out, try some new sports, and most of all, have fun!