This summer, my mother-in-law bought a suminagashi marbling kit (similar to this one, which only costs $13.50) and all the grandkids couldn't stop using it.
This activity is so simple that even T didn't need much help (though be careful, the dyes stain). All you have to do is add a few drops of the suminagashi (what a word) paint to a pan of water, then swirl the paint with a chopstick or barbeque skewer. Dip a sheet of watercolor paper into the water and the results are absolutely gorgeous. When you want to make a new design you don't even need to change the water in the pan, instead just use a strip of newspaper to soak up the old colors. Easy peasy.
Of course, now I need to figure out what to do with over 50 sheets of marbled paper. Make greeting cards? Wrap very small presents?
I think this would be a great activity with a large group of kids, as you're pretty much guaranteed beautiful results and the set-up isn't very complicated.
If you don't want to invest in the suminagashi dye, we've also marbled with:
Post by Darcy : No Monsters in My Bed