Have you ever tried making Magic Mud? It's just cornflour and water, but it's great fun. Technically, it's a non-Newtonian fluid; it acts like a liquid when it's being poured and like a solid when force is applied to it. In practise, it's just great fun to play with. If you're a QI fan like me, you may have seen Stephen Fry making it dance on top of a speaker!
To make enough for 2 little monkeys to play with, I started with a cup of cold water and gradually added just under 2 cups of cornflour, mixing first with a spoon and then with my hand. It tends to dry out if you leave it, but you can always just mix in a bit more water and you're away again.
The real beauty of this is that it's a lovely messy way to play but so easy to clear up. I stripped the boys off, but it really wasn't necessary, as it's completely washable. You can easily wipe up spills with a cloth, and if you leave it to dry you can simply brush it off. Next time I wouldn't even bother with the splash mat; it's easier just to wipe the table. You could add food colouring if you wanted pretty colours but as far as I'm concerned that just makes clean-up more of a hassle, as it tends to stain little fingers... Anyway, the boys loved it, plain white and all. T had had enough after about 20 minutes but K played with it for an hour. It would have been even longer if we didn't have to go out after that.
So if you're still on summer holiday, bring out the cornflour!