I am or have been part of:
-UCSD Photo Club - Photography enthusiasts unite and go on quarterly trips to scenic locations to shoot
-Alpha Phi Omega (APO) - National co-ed service fraternity
-Nikkei Student Union (NSU) - Japanese-American culture club
-Foreign Film Society - Org that screens international films at PC for free!
-Public Health Brigades (now known as Project Nicaragua) - PH-based org advocating for the narrowing of health disparities in developing countries / volunteer abroad
-Psi Chi - Psychology Honors Society
-Omikron Delta Epsilon (ODE) - Economics Honors Society
-Undergraduate Investment Society (UIS) - Pre-Banking Org
-Actuarial Society - Pre-Actuary Org
The first five are/were great. The people who join, for the most part, are in there for fun and to meet people and I definitely recommend them to anyone if you want to just have a good time. Just look at
StudentOrgList for an up-to-date list of REGISTERED orgs. Orgs have up until week 4 to register through SOLO as an official organization, so it's still getting more and more groups listed there daily. Find something that piques your interest and just go to a meeting or two and see if it's worth continuing. Orgs aren't mandatory and you don't have to make any commitment (except frats/orgs that have fees/dues).
The next two are Honors Societies; if you can get in them, they are of help for you socially and academically. Though graduate school (whether or not you're considering them at the moment) don't care so much that you're in them, but they do provide valuable resources to you as an undergraduate. Professors give informal talks or will advertise research opportunities with you first, as you're the supposed cream-of-the-crop of that department. There are student/faculty mixers where you get to know your professors outside the classroom--for ODE, there was a mixer at Porter's Pub with beer for those who were of age, for example. This is a fantastic way to start building relationships with faculty who can then speak of your personality and perhaps write letters of recommendations later on when you're ready to break into the workforce or apply for school.
The last two, being pre-professional orgs, contain a lot of people who really just go to boost their résumés it seems. They're a lot 'clique'ier than what I would have liked, with your ability to make friends being contingent on your alleged intelligence and/or work experience. Basically, if you're a scrub, it seems that no one will want to network with you.
All in all, you have your interests and I have mine, so definitely take my post with a grain of salt. I really highly recommend doing Photo Club, though. I'm friends with a few of the board members and it just sounds like mad fun. No expensive dSLRs are necessary, either! Some people just go to trips with a compact P&S camera