Sports teams are pretty big. The Programming Council and Student Government Association are also popular, I think. There are so many groups though that I don't really know what is most popular. I am on the Ballroom Dance Team, in the co-ed fraternity Phi Sigma Pi, a volunteer for the Boltwood Project, and sometimes a dancer in the student choreographed dance show. My closest friends are those from my freshman year dorm, plus some of my hometown friends, though I am getting closer to the people in the groups that I am involved in.
Greek life isn't huge, but relatively present on campus.There are always dirty frat parties happening on weekends, but there are other things to do as well. I hardly ever go to them. There are a few frats that are not simply social. APO is a coed service frat, and my frat, Phi Sigma Pi, is a co-ed honors social/service/academic fraternity. I wasn't originally interested in greek life, but PSP is service-oriented, doesn't haze, and is an overall friendly and welcoming group that I've happily found a place in.
As for the other things to do, UPC has friday night movies; there are acting groups that give shows, performances at the FAC, theatre productions, concerts, etc. There are tons of faculty lectures happening all the time in every department it seems. CommColl has weekly lectures and workshops.
Off campus has plenty of opportunities: movies, shopping, eating, shows and concerts, rallies, museums, etc. Basically you can always find something to do here.