There are sooo many things you can do with old crayons! I'd be having a heyday if I were you.
I really like the candle idea, but I'm a candle fanatic so that might just be me. If you have kids, you probably don't want tons of candles hanging around, although I'm sure they would make great gifts.
Another idea would be to frame them! Make a cool abstract art piece by placing them on the inside piece of a glass frame. Fit as many as you can, you can use all different colors or stick to a theme, and then put the cardboard backing on and voila - a neat piece of art.
Put them all in a see through plastic vase as decoration in the kids room.
Melt them down and make heart shaped multi-colored crayons for the kids at school on Valentines day.
Just a few suggestions!