I suggest an abacus. It's a fun way to concretize mathematic operations. You can learn to represent up to ten digit numbers on the abacus, then do addition, then multiplication and division. Once you're multiplying numbers you can start to learn shortcuts to make using the abacus faster.

I think the abacus is a fun addition. We keep one on the counter where we eat breakfast. It's fun to just pick two three digit numbers and multiply them out.