kidsbowlfree.com and sign your kids up. I did it yesterday, and it's brilliant. Your kid can bowl two games free every day of the summer and all you have to pay for is shoe rental.
Another source for cheap summer entertainment is the dollar movies. Check out the theater near you and see if they offer a summer kids' program. We printed out the schedule and picked the shows we want to see.
Also, check out your local library for summer programs. Ours is doing a reading game, and they offer weekly entertainment: magicians, animal shows, etc.
Just a few ways you can beat the heat this summer. What are your favorite summer secrets?