If you want to design with them, then color on sandpaper - fairly thick on the rough side. Then turn the sandpaper over on top of fabric like a plain T-shirt. Iron the back of the sandpaper (the smooth side), and the colors will transfer to the fabric. You have to experiment some to get the heat right. The thickness of the sandpaper and how thickly you colored on it affect what temp you need to have on the iron.
The melted crayon art works great by running the crayons through a hot glue gun - use the crayon in place of a glue stick to melt it. It doesn't even ruin the glue gun - although I had to run an entire glue stick through it to clean the crayon colors out of it.