One of the ideas on our Summer of Fun board was to make some candles out of sun-melted crayons. As it happens, being a stinking hot summer and all, the sun and accompanying heat was happy to work with us.The original idea is here, but I made a couple of adjustments. Rather than mixing lots of different colors, we kept like colors together (for the most part) and put them in baby food jars. Rather than getting proper candle wicking, we went with the suggestion to simply put a birthday cake candle in the middle, which worked just as well.
We had them at the table for dinner and the kids enjoyed having the candles "race" - they don't burn especially well, so it was a competition to see who's would stay lit the longest.
For us, it was a fun afternoon activity when it's too hot to go out. If you wanted to, you could make it a cool solar energy/color combining activity, as well. It was too hot for me to be clever about it, though!