We recently bought water beads from here, really cheap to buy, just takes a bit longer to arrive in the post. If you haven't used water beads before then I highly recommend buying some to try them out. They turn up in tiny packets of 'seeds' and when you add them to a bowl of water they 'magically' grow to marble sized beads. The original purpose of the beads are to use in vases of flowers, partly for the flowers to absorb the water from the beads, but also for decoration - they come in many, many colours.
There are so many ideas floating around Pinterest, from painting with water beads to role play. For younger toddlers the simple emptying of beads from one container to another is great fun, the feeling of running your hands through the wet beads is also a delightful sensory experience for kids of all ages. Anyone passing by our bowl of water beads has to dip their hands in!
O loves pretending to cook with his play kitchen pots, pans and utensils. Stirring a big bowl of beads with a large spoon is also apparently quite mesmerising!
Disclaimer, the beads are not edible, please keep an eye on small children who still mouth items.