We have had this super fun book out from the library recently, Math Lab for Kids by Rebecca Rapoport. It has so many cool activities in it. My daughter (grade 4) has loved working through them. Unfortunately we had to return it before she got through all of it, but we plan to get it out again. It is really attractively presented with nice illustrations and clear instructions. I highly recommend it. Apart from me needing to help her get materials occasionally (and hold the odd broomstick to draw large circles and ellipses on the concrete outside or support a collapsing dodecahedron) she has also been able to most of it on her own, which is great! Here are a few things she has made.