Instead of sand, we used oatmeal. Lots and LOTS of oatmeal! I love using the oatmeal instead of sand because I did not have to worry about Clara eating it, and it is much easier to clean up. You can find large containers of oatmeal at the dollar store or places like Big Lots. This pool used about $14 worth of oatmeal to fill it up.
The girls played in here for several hours, and we will use the empty containers for future projects. When they were finished we dumped all the play oatmeal into a large plastic bin (to use for more projects) and deflated the pool.