The Field Museum, Adler Planetarium, and MSI are all free this week. I figured I would head down to the Field tomorrow and then maybe walk over to Adler. Of course, the free admission doesn’t cover the shows. I might see one if I really feel like it, but otherwise is there much to see?
I don’t know much about the Adler, but I would warn you about free days: get there EARLY! Free days bring out lots and lots and lots of people. It’s almost not worth it.
I was last at the Adler in 2008. They have some science-museum type galleries about the solar system and the history of astronomy, and the Atwood sphere is pretty cool. It’d probably take you 1–2 hours to see everything outside of the planetarium shows. If you’re interested in space and astronomy, it’s worth checking out.