Once again, the Smithsonian is offering free admission to various museums this Fall.  There is a limit of one pass (good for two people) per household, and you must request your tickets in advance.  More information is available here:  http://microsite.smithsonianmag.com/museumday/

This is going to make a great cheap homeschool field trip!  There are quite a few participating museums within driving distance of my house!