The Chicago Architectural Foundation [[or something like that) gives Loop Tours on the L, or at least they used to. Tickets are free. Very educational and informative.
In DC, the obvious pick is to bum around Union Station for a bit. A Metro stop away at Judiciary Square is the National Building Museum, which is not only an interesting museum, but one of the most beautiful interior spaces I've ever seen.
In both cases, you can check your bags in a storage room at the station [[I don't think they allow you to use the lockers anymore).
Have a blast!
Bookmarks