Ummm. Wow. I was going to point out the trolley service for downtown but it has been discontinued for 2009. Damn. I loved that trolley.
Unless you're walking to Navy Pier (which I wouldn't advise) or taking a taxi, the only other offer they have for transportation assistance is this:
Navy Pier Trolley
Navy Pier offers daily free trolley service between Navy Pier and State Street along Grand Avenue and Illinois Street.
Which means you'll either have to walk to State Street or Illinois Street (both are still quite a walk).
Damn damn damn.
Chicago Metra offers a $5 weekend pass. You can ride in and out of the city as much as you wish, as long as you don't lose your ticket. This will take you to Union Station.
From Union Station, there are still many things within walking distance. Sears Tower, Art Museum, Millenium Park, and Michigan Avenue (lots of shopping there).
What you decide to do will depend on what your likes and tastes are. I can try to help answer questions if you have any.
Ditto that you'll be doing some walking...from Ogilvie Station (the Metra stop) to the Art Institute/Millenium Park is a mile, the Field Museum is roughly 2.5 miles one way as the crow flies. If the weather is good it's very pleasant (so much to see!) but you might want to have some cab fare handy if it's crappy out.
