I tend to stay inside all summer, because the car gets so hot if I go anywhere cool, all the relief is gone by the time I've driven home! So I only go out at night. That wouldn't work with active girls, though.
Around here, a lot of people go to the mall because it is air conditioned. Those play areas creep me out, but if they are reasonably clean where you are I'd consider it.
Other than that, can you get a kiddie pool and splash around in the yard? Set up a sprinkler and run through it? Wash the car? Take a really long bath indoors? (I guess I'm stuck on water).
Me (40) DH (47) & furbabies * m/c 7/08 4/12 11/12