There are several dance clubs on campus that give free lessons and dances, which is awesome. There are lots of hiking/outdoors-y clubs. Lots of martial arts clubs. I don't know what the most popular clubs or teams are, but we have pretty much whatever you could want, from basketball to Balboa.
A group I'm involved with is SPS--Society of Physics students, which hosts meetings once a week for an hour, includes food and drink and usually a really interesting speaker. Great for physics students, all though ironically a lot of my classmates don't show. Some are shy, some just aren't interested, some are busy.
Students do NOT leave their dorm rooms open. Stuff get's stolen a lot, though it also gets recovered a lot.
Athletic events don't seem to be the most popular, though they certainly happen. Lots of guest speakers, on everything from masculinity in hip-hop to gender roles in Peru to the newest information on supernova. We have a great selection in that regard.
The dating scene...I don't really know. I met my current boyfriend here, through an activity, and people do seem to pair off, so I'd say it's reasonable.
If I'm awake at 2am on a Tuesday, I'm probably studying.
There are a couple of cool traditions every year. One is 4/20, called "four twenty" which is on April 20th when a whole bunch of people get together on the Porter meadow to smoke pot. I didn't even hear about it until I came here, probably because the administration is really embarrassed by it. It's the only event I know of where it's really obvious that people smoke pot on campus. Otherwise people keep that sort of thing mostly to themselves. There is drinking and such, but I've never heard of a lot of hard drugs being done on campus. It's a very "let's drink healthy organic drinks and eat vegetarian and smoke pot and save the world." By the way, you can find pretty much any vegan or vegetarian food, either around the campus or on it, since Santa Cruz is so hippie.
Another tradition would be the Vagina Monologues, put on by students at Porter college every year for Women's Day. They do a very excellent job.
People who party on campus are mostly freshman and sophomores. The major partying goes on off-campus for juniors and seniors. The partying is not evenly distributed. At Crown there's hardly ever any, at Porter and Kresge there's a lot. It depends on where you live.
I don't think fraternities/sororities are super important, but they're around. Rush week just happened, so all the Greeks came out to try and recruit people. I'm not really interested, and people didn't seem to be breaking down the doors to sign up, but they definitely exist.
Last weekend I went out downtown for pizza with friends, saw a movie, and studied, and slept.
On a Saturday night you can go downtown, see a movie, eat good food (my one complaint is there really isn't any great Indian food around...at least not like in San Francisco), go shopping, visit the Boardwalk, or hop on a bus and go to Capitola and hit the mall, or just stay in with some friends and play some board games. If you like dance, there are some local social dance venues, too.
Off campus I do all of the above, go to the beach, hike in the trails, go dancing at local venues, etc.