MagnaTiles

This is a wonderful, long lasting toy. We bought this for our kids several years ago and we still use them. You can make an endless amount of structures and buildings easily. They are all attached magnetically and you can arrange them in almost any way. You can combine shaped to make other shapes that you want.

I recommend these starter sets:

MagnaTiles 32-Piece Starter Set This set includes: 2 large squares, 14 small squares, 8 equilateral triangles, 4 right triangles, and 4 isosceles triangles. This is enough for one kid, but if you have more you will definitely want this:

MagnaTiles 100-Piece Set This set includes: 4 large squares, 50 small squares, 20 equilateral triangles, 11 right triangles, and 15 isosceles triangles.