My kids enjoy sand boxes, but sand gets EVERYWHERE and it is a pain to clean up - not a great combination for indoors. So, substituting rice for the sand has worked out much better. This project is also very cheap to make because you only need a few supplies:
1) Under bed storage tote - Any tote or container will work for this project. I picked the low profile of this tote because I wanted to be able to easily store it out of the way. Also, I first made this when my kids were still very little, so I wanted them to be able to reach in easily.
2) A few big bags of rice - The amount will depend on how deep you want the rice.
3) Shower curtain liner - I can not emphasize enough how vital the shower curtain liner is when it's time to clean up.
4) Toys - Any toys that they already have. I would recommend toys that don't have openings for the rice to get in. Otherwise, the "toys" will become "rattles".
These are fun places that I like to share - have you checked them out yet?