I was wandering around the blogosphere today, and I stumbled across this excellent list of free summer activities at Couponing 101: Free Fun All Summer Long. Stephanie's compilation includes museums and parks, workshops, movies, books, and other fun stuff. There is so much to do, I don't think that summer is going to be long enough.
If nothing on that list excites you, call or stop by your local base recreation or community services offices. I'm always amazed at the number of events going on each month. Between the military community and the civilian world, you are sure to find something to suit your situation.