Program a robotic arm / crane thing that can teach sorting algorithms, data structures, and fun math problems!
Most of files for 3D printing can be found- https://www.thingiverse.com/thing:1015238. Others, including magnetic attachment and blocks are in /models.
Grid was drawn on paper, each square is a little bit larger than a size of a box. Small magnets and washers should suffice for picking up blocks.
App example contains bubble sort.