One of the best tools I have found are these fabulous "Rain Beads".  A friend of mine discovered these beads several years ago.  She brought them to school one day and shared them with me.  Up until that point I had been using sand in my multisensory trays.  


The students LOVE the beads and we no longer use sand at all.  

The beads feel so much better!

We've had such good results with the beads that we now sell them.  $5 for a 16 oz. bag.  Each bag will fill at least 2 trays.  Well worth the money : )  

P.S. I have a few OT friends who use the beads in buckets for sensory activities.