I have done this several times through out Caleb's life, starting him quite young. I make sensory tub's filled with water beads. They feel fantastic on your hands and fingers, they are tiny beads that soak up water and become larger. Super easy to do. Here is how I did his Summer bin:
Use a tub to hold the beads and water
I purchase these beads here on amazon for only 1.77 a package, I used two packages for this tub and bought a few extras for next time
Open the packed and empty in tub, read the directions of how much water to use.
Usually takes around 1 hour to bloat up
I then purchased these summer play animals here on amazon as well
That's it, simple and fun play. I don't believe these beads are toxic, as I originally got the idea from other Pinners on Pinterest. They may be a choking hazard for younger ones and you should keep a eye on them when playing with these.