Found this great project: http://www.5orangepotatoes.com/blog/2011/03/04/hot-rocks-painting-with-wax-on-rocks/
We had a great time watching the crayons melt on the hot rocks. I cooked the rocks at about 375 for probably 20mins. The two big rocks stayed hot for a really long time. K's smaller rock started to cool off before he was really done.