First, there are sprinkler toys - the crazier the better. Melissa & Doug Sprinklers which run at $19 are great for the little ones. Or more ballistic models like the Geyser Blast at $32, may entertain the slightly older kids.
Next, if you have access to a pool or a lake, check out the Waboba Extreme ball. The ball, which bounces on water would definitely keep a 10 year old busy. It is made of some crazy space age polymers, it floats, and it only costs $11.
A rousing ping-pong rivalry could chew up a little summer down time with the Sportcraft "Anywhere" Table Tennis set. For just $15 you can turn any table into a makeshift ping-pong table.
Finally, a waterslide is great for kids and grownups alike. However, don't spend a bunch - you will lose the pegs, it will tear, and it is s a one season only purchase. Waterslides are more fun if you have a hill, but you can build up some speed on the flats if you lube up with sunscreen or even better, Pam cooking spray!