I'm considering a fly in-and-out visit of Chicago this summer. I would get in between seven and eight in the morning and leave about five in the afternoon (for a seven PM flight). Any suggestions for how to spend my day?

Here's the parameters:
*I'd probably be flying into O'Hare since they have the earliest in and latest out scheduled flights of the day via AA. I would be open to flying into Midway via NWA if someone has a really good reason to do so.
*To save time, I don't want to bother with a rental car. Taking the El and/or buses is not a problem, nor is walking.
*I will be either alone or possibly with one other adult.
*I'm an art, architecture, and history lover (The only must-do on my mind is to see the Art Institute.)
*I do not want to spend any time shopping or with significant numbers of children.
*Food is not a priority. Yes, I know that they have good pizza in Chicago but I don't really care about it. I'd rather spend my time seeing the sights than sitting around a restaurant. Packed sandwiches and McDonald's type meals would be the extent of what I'd need.
*If you think that the view is worth it, should I visit the Sears Tower or the Hancock Observatory?

TIA!