Md's playland: yes I realize for some of you this means having to buy food, but go in the afternoon for an ice cream treat (mc flurries now come in snack size for under $2) and let them run out the post nap wiggles
park district pool: at this point in the summer memberships are normally at least half price, but even if you just pay the daily rate, going later in the afternoon could offer a discount. Our local pool cuts the daily rate in half after 3pm. The pool closes for lessons at 5, so that still gives you 2 hours of fun in the sun :)
movies: don't forget that Wehrenberg Family Summer movie Series
There is always the dreaded Chuck E. Cheese; head out early and beat the big kid afternoon rush!
Check your local library out. Spend some time wandering the kids section and read the books there before checking them out. Many libraries offer summer reading programs!
When all else fails, have a playdate and keep the kids in the basement! That's what the Wii is for no? :)