Shesus mentioned some awesome stuff.
I'd like to add that if you're architecture, make a trip to Wilmette and visit the Baha'i temple. The surrounding area is right on the lake and is always a great escape for me on warm days. Plus, there are only 7 Baha'i temples in the world and the only one in North America happens to be in Wilmette, about 4 miles from where I live.
They're doing some construction, however, so about half the gardens surrounding the temple are closed off.
If you go in July, Grant Park has free movie nights. It's awesome. Good to spend some romantic night outdoors overlooking downtown to a good movie.