Go to the top of the Hancock building, but don't take the tourists elevator for $10. Instead, take the bar elevator and spend the $10 on a nice drink. Second City is the comedy club of choice, and of course the Shedd Aquarium is awesome. You could spend all your time at the museums if you were so inclined.
As for info on clubs and restaurants, check:
Chicago City Search
I live in downtown, and I'm not much for the club scene, so I can't provide much useful information there. The bars I enjoy going to are Irish pubs, like John Barleycorn, Timothy O'Tooles, and Fados. House of Blues typically has great, local live music in their bar area. Near HoB is a bar called Rockit, which is pretty popular with the younger crowd.
The club scene is really hopping in the Rush and Divsion area. If you get up there by cab or whatever you will be a stone's throw from probably 20 clubs. Lincoln Park and Bucktown/Wicker Park neighborhoods are also really cool places to go to for good food, shopping, and night life. Wrigleyville (south of Wrigley Field) is a great area for casual bars as well.
Don't be afraid to use the busses/trains to get around, and be ready to do some walking unless you don't mind spending money on taxis. I would buy a five day pass if I were you, and then use it for everything but getting to the club at night. Nicer restaurants tend to be pretty expensive, so check menus first. It is pretty easy to drop $20 for dinner without drinks. Club's can be pretty expensive to get into, so keep that in mind. Look for places that are offering a good drink deal early in the evening. Some places will do a $20 cover for all-you-can drink between 9 and 11 or 12.
Other things to do:
-- Walk down/up Michigan Avenue. Even if you don't shop, it is pretty cool to see all the stores. The Apple store is awesome.
-- Go to the Lincoln Park Zoo. It is the perfect way to kill an afternoon, plus it is free.
-- Take a tour boat up the Chicago River. Another good way to kill an afternoon, plus you learn a lot about the city.
-- For Chicago-style deep dish pizza, I recommend Gino's first, Giordanos second.
-- Don't bother with Navy Pier unless you want to take in an IMax show. It is a total tourist trap for people with kids.
Where are you and your buddies staying?
R