Not sure of the ages of your children, but there are lots of activity options!
Visits to the library are educational and free. And most libraries have summer reading programs. Many libraries also have museum passes you can use to go visit the local children's museum, art museum, science museum etc. for free or very discounted.
And lettign your child play outside can also be educational! I learned a lot as a kid just by spending time outside with my neighbors. We managed to build and create forts, rescue a small turtle, and learn about all sorts of nature things. Planting a garden could also be very educational. The kids could help with watering the plants, weeding etc. and learn about the growing process! Plus kids are often to eat that "gross broccoli" or "yucky squash" when they had a part in helping it grow :)