DIY Sand and Rock Box by FSPDT. Gdjk -> made this with toilet rolls (aka tunnels) and moon sand instead of play sand. My son loved playing with it. The moon sand cleans up nicely indoors but play sand will be easier to play with when using the small cars.
This is another example of exploring math through loose parts; but in contrast to the other one, this one uses materials with very similar shapes, sizes, and textures and only varying in regards to colours.