Some ideas might include indoor water play. It can be nightmarish, but you might be able to pull off all three of them in swim bottoms in the bathtub (or naked). Let them have a strictly play bath with "no flooding" rules and put all the weird toys in your can think of. My boys love playing with their dinosaurs in the bathtub and they are not hollow so I only have to rinse them off.
Alternate your table crafts--melt ice cubes for the older child on construction paper and the younger ones can paint with water on colored construction paper. Assemble a lot of glass jars on the table with water and have "experiments" with dropping food coloring in. Let them guess what color you can make and watch the colors swirl (I did this around meal prep time when I worked with 12-24 month olds in child care and they loved it).
Do "science" projects with baking soda and vinegar, or put colored pieces of tissue paper in glasses of water.
If your furniture is arranged around the perimeter of your living room, put dining chairs in the gaps and let your kids go mountaineering. (Mine do this anyway and I have not figured out a realistic way to not let them--it is often too hot out here for them to be out much, too.)
Make a countdown to July 4th by having them each make a flag or make a different kind every day with different materials. They could make a collage flag with torn paper, cloth, crayons, paint, odds and ends they find, or color in a prinout. You can teach them the Pledge of Allegiance and, if you really want to give yourself a headache, "You're a Grand Old Flag." (A 3 year old I babysat sang that for about three months--it got old--and my older son loved saying the Pledge every time we saw a flag from the time he was about 2 1/2.)
Find a couple of moms in your area who also have air conditioning and see if you can have one "exchange" morning or afternoon and play at the other's home. The change in company and a novel play area can be great for kids.
We like to recreat small habitats. If you can find a 10 gallon aquarium at a garage sale or something you can let your kids go out in early morning or early evening and catch things and then find things that belong in that critter's habitat and try to set it up. Then you could do some research with them online to learn about them and let them go after a few days.