There are a couple low-maintenance crafts we managed to complete this summer. Ethan came home with the first idea from his end-of-the-year party at school. He begged and begged for us to try it at home. Fortunately, it's easy and cheap. Find a nice, smooth rock and heat it up in the oven for about 15 minutes at 250 degrees or so. Peel some crayons in the meantime. Take out the rocks and color them. The crayons melt and create interesting designs. Not great for Adam who would probably burn himself but fun for Ethan and Drew.
Another old favorite was perfect for this hot summer. Freeze water in layers and add little items (pennies, legos, beads..) every few hours as you add water. Bring it out on a steamy afternoon and you'll be amazed at how long this keeps kids occupied.