Besides what has already been mentioned? Watching funny movies or good things on TV like CSI or Law & Order usually gets me smiling. Good Will shopping is another good one... I rarely have money to burn on new clothes, but I'm always surprised at how much awesome stuff I find at the thrift store. I also listen to music while stressed. Angry punk music usually does the trick for me
I recently bought a punching bag, which has also proved to be INCREDIBLY liberating. I thoroughly enjoy punching and kicking the crap out of it and often feel MUCH better after I'm done.