This great toy lets kids feel the heft of a piece of metal in their hand, and they work on fine motor skills, organization, planning, sequencing... plus it's fun!
My husband picked up the case, nuts, bolts, and screws at Home Depot for under $20. You could even pack it less full.
The preschooler was given this toy with the express instruction that if he left any part of it out of its case, or let his [baby] brother touch it, it would disappear. He has been fastidious.
Thanks to my friend for the idea (her son was playing with one and we had to go make one).