Had when i worked in a preschool and kids love them! I would use colored sand instead of white salt. DIY salt tray with alphabet cards. Easy to make and kids have fun smoothing out the salt after correctly making the letter
To make a river stone mat first you have to gather or purchase river stones. After that take a plastic mat and arrange all the stones on it. Then stick all the stones one by one to the plastic mat by using silicone sealer. Let the sealer dry completely.