There are several museums in the Puget Sound area that offer free admission on the first Thursday of the month. I plan to visit some of them this school year. Let me know if you know of any others.
Museum of History and Industry in Seattle
Burke Museum at the University of Washington - Seattle
Washington State History Museum - Tacoma (free from 5pm-8pm)
Seattle Museum of Flight - Seattle (free from 5pm - 9pm)
Seattle Asian Art Museum - Seattle (free on First Saturday too)
(the Seattle Art Museum usually has the First Thursday free too, but they are currently closed for remodeling.
Wing Luke Asian Museum - Seattle