You can get 1kg of rice from most supermarkets for about 40p. Alright, its not the best rice for eating (very starchy) but its ideal for the sensory tub and the children would much rather play with it than eat it most days.
Pour a couple of bags of rice into the tub and let kids go wild. Give them some funnels, colanders, pipes, and big spoons. Plastic plant pots are great too. The kids will play with it like sand, its just a little easier to clean up when the throwing starts. This time we gave them some old socks to fill up and they played with the “little feet” for ages.