Can differentiate with double digits (double digit by double digit multiplication or even fractions). This site has great math projects. This can be done with nails in a board and colored yarn. Makes a great pricture too.
How To Easily Cut The Bottom Off A Wine Bottle -- (I would try this outside first. Don't know if there are any small fractures in the bottle when the 2 pieces split in the cold water. Interesting idea and definitely worth a try.