DIY Portable LEGO Kit with Free Printable Activity Cards. I like this for school, older kids can have activity cards to create planets/atoms/molecules/fractions, younger ones can practice fine motor skills.
Could also connect a bunch of locks and put all the keys on a key ring. Lots of pictures of latch, lock, etc ideas for montessori type preschool. This lock one could be made with 2 boards, some o-rings, and some locks from Dollar Store.
Counting and Measuring with Lego: Preschool Maths Game - Good way to visually compare numbers - I would suggest making the corresponding numbers (like 5 and 15 or 8 and the same color to help see the pattern relationship between the numbers and 15