Calming bottles - these sensory bottles are great for little ones to explore and for preschoolers to use during a cooling off period. Water bottles with small amount of water poured out. Glitter glue, loose glitter, jewels and superglue to seal cap.
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.