A+
Varied, Lots of Options
This varies whether you are Greek, rich or friends with a lot of upperclassmen. If you are not Greek, there are a few open parties available, with usually at least one open party during non-exam season (though this might decrease now that Pike and AEPi are shut, no one went to Psi U anyway).
For those who know people in upperclass dorms, East Campus is a common destination though the parties there tend to be a little cramped at times.
If you are rich and have a good fake ID (and I mean, really good, scannable and blacklight compatible) then you can venture downtown to the clubs which obviously are incredible, if a little pretentious at times. However, this requires people who are similarly wealthy/ID holding. The latter is obviously irrelevant when you turn 21.
There are several bars around campus that are relatively easy on ID (though you will still need one, it just means they don't have to be very good, photocopies sometimes work). These include Mels, 1020, the Heights (bouncer here stricter than most), and Cannons to name a few, each attracting their own kind of crowd.
Finally, if you are Greek, if you're a guy you often hold things (mixers, informal gatherings) at the house, and then usually end up at a bar afterwards.
So, overall you have a lot of choices, and often you'll have to pick whether to go to EC, frats, Mels etc etc in one night. Fake ID is a must though.