I bought these tiny rainbow beads online - (you can also use the silica beads that come with trainers ) a couple of pounds for thousands of them.
All you need to do is pop them into a container of water
Simple activities can include counting , how many different colours, holding the container up to the light.
Pop a hand in:
How many am I holding? Two?
Further activities can include timing how long they take to grow, if different colours take more or less time. Can also see how long they take to return to normal size.
How about trying a salt solution? Or pop some food colouring in.
Lots of potential for measuring , weighing and if you have a torch handy see how the light is refracted by the spheres. See what happens if you hold one above some writing.
If you've tried this, let me know how it goes.
**Important** don't throw these down the drain when finished with..they can be dried and reused over and over . If they must be thrown away they'll need to go in the general household waste.