Smithsonian Magazine has announced a free Museum Day on 9/25! There are tons of museums participating (including the Adler Planetarium and Museum of Contemporary Art among others in Chicago), so hopefully you will find something in your area.
In order to get your ticket, fill out the information on this form. Your ticket (which will be emailed to you) will give both you and a friend free entry. One ticket for two per household.
(Thanks Freebie Fanatics!)