These are things for older kids, but:
1. Make 2 models of the solar system.
A. Find the diameter of each of the planets and the sun and figure out how big you'd need to draw each to be able to cut and paste a model of the sizes on your bedroom wall. (Note: The Sun will take up an entire wall and dinky planets like Venus and Mercury will be itsy little things.)
B. Find the distance between the planets and figure out how many inches to millions miles you'd need to make a distance model of the solar system. Measure out in string inches to millions of miles between planets, putting markers at each planet. Roll up the ball of string and go outside and use a block or two to unfurl the string. This will do more than anything to impress upon a kid the distances between the planets. (Even adults will go by with this look of "Dang! I did NOT know the solar system was THAT big!")
I am told you can do similar things with molecule models made out of fruit and toothpicks, but chem wasn't my thing so we never did that. . .
2. Make your own pottery.
You can find websites and books that describe how to find clay on the banks of creeks, then turn it into the kind of clay or slip that's used in making pottery (beware your current pots - you need grog, ground up old pottery, to make new pottery!) and then fire it in a garbage can kiln. This will pretty much take up an entire summer if the kids are interested -but it's only for older kids. Still, it's satisfying to turn clumps of dirt into slip through long and tedious watery outside processes, and then form pots using all the various methods (ball and coil mostly) and then try to fire pots (many of 'em will shatter in firing, but that's part of the fun - you've made GROG for your next attempt!)
3. Gardening expirements
Buy beans and corn. Put them in papertowels and see how they sprout. Put them on the glass facing of water glasses, shoved in with papertowels and watch the little plants grow.
buy sweet potatoes, pineapples, potatoes, carrots - you can grow plants from all of these as well. And, you can plant them in flower beds and watch 'em grow!
4. Make your own movies
Collect old costumes, go to thrift stores for old formals and old wedding dresses and fun clothes. Set up a video camera to a computer and let the kids make their own movies. They'll have to learn scripting and shooting and acting and dressing up. This can also devour entire decades of summers. (Warning: You'll have to sit through the masterpieces. Several times. And break up catfights between budding little starlets who all think they should be the romantic lead!)
5. Buy a Wal-mart pool
Kids will swim in these things all summer if allowed to do so.
6. The library is your friend. There are great programs and it's all FREE! Plus you can pick up a ton of books full of good ideas to make the summer fly by.
No, I don't have kids, but I've babysat and babysat and babysat when much, much younger. I didn't want MY summer ruined, so I figured out fun things that took up entire WEEKS of time that I didn't mind doing with them. . .