Cool, you should be able to find whatever it is you're looking for..
I like the lake, beaches, path, volleyball, sailing, but only on the weekdays..
If you want to experience the city 'chicago style' you should try ditching the car at some point, especially if you want to do night-life stuff at peak times. Try the L/metra/cabs/cta/bike/walking. Biking is pretty much faster than using anything else, but it might not be for everyone since you'll be riding in (somewhat stationary) city traffic and more importantly watching for car doors in parallel parking lane.
For college age, Northside area like wrigelyville is full of hip people, nightlife, crazy clubs, social activities, bars, coffee shops, poetry readings, backyard barbaques, whatever. There are many good local musicians. There is also tons of theater but there are many acts that are contemporary/experimental.. There are many block parties and events, but that is starting to taper down now.
In the actual loop you can do restaurants and shopping, but it turns into a ghost town at night.. Oh, there's lots of history and architecture in the city, you can take boat tours, etc, of it.
The city is also still somewhat organized into the historic ethnic neighborhoods, so you can see little italy, greece, china, etc.. which will each have their own style, shops, restaurants, etc.
Didn't know lincoln park zoo was free, I've gone by lincoln park zoo so many times, but never tried to go in. Hmm, I've also never been to the Sears Tower either. I do like gino's east/lou's and like mustard and no ketchup on italian sausage.