You'll be here for Taste of Chicago!! I don't know if you can go during the day but it would be so worth it to check out, and its free unless you eat (which you definitely should)
A lot of people your age hang out at Navy Pier on like, Friday and Sat. nights.... I just turned 21 so trust me Stan!!! GOD i have gotten really inventive having fun in the summer here, since clubs are my kind of atmosphere...
If you have some $$$ on you on Thursday nite, the Keg in evanston does not card. Well ok they do, but eveyrone in there is like high school age and you can give the guy 10$$$ at the door and he'll let you in.
I also have friends that work at some really cool bars downtown- PM me if you want me to give them a call and see what they can do