Navy Pier, Chicago. Been once but MUST go again. Didn't get to do anything there last time due to a particularly loserish ex.
Took a stroll down Navy Pier - Chicago. Walking and taking in the beautiful view is completely free - individual exhibits cost, but completely worthwhile just to watch the boats in the harbor and the people (of course!).
A Chicago Water Taxi is a great way to get around and gives you a different perspective of the city. Try taking it from Navy Pier to Chinatown.
Love from Chicago: Skyline Notecard Set
chicago skyline silhouette with navy pier - Google Search
Google Image Result for http://upload.wikimedia.org/wikipedia/commons/thumb/a/a2/Navy_pier.jpg/250px-Navy_pier.jpg
Architectural river cruise - best way to see Chicago for the 1st time. That was a blast doing would do again in a heartbeat!!