If you have a Bank of America credit card, ATM card, or check card, Bank of America offers free admission to over 120 museums the first weekend of each month (which is this weekend). Click here to find a museum near you.
Bank of America has added over 40 new institutions and more states to the list (the year runs May 2009-December 2010), so if you didn't find anything you were interested in before, you might want to check out the new list. Also, make sure you check the list, since some museums don't offer free Sunday admissions.
We were planning on possibly going to the zoo for free this weekend through this program, but we found out that the zoo has been discontinued this year. However, they've added 2 new fun museums that we've never been to. So, I'm sure we will try one of them out soon (if not this weekend)!