I would set the kids up with a giant plastic tablecloth, tons and tons of plain white paper, and many tubes of glitter glue or paints. Making Rorshach prints kept them busy for days (glob on the glue, fold the paper in half and look at what you get!) String up a clothesline outside so the pictures can hang up to dry and they can get to work on more.
Shorter term, they liked to play with food coloring. Fill a shallow bowl (the lids from takeout containers work) with whole milk or cream. You don't need a lot. Let them drop a little food coloring into the milk. It'll just sit there because the milk is oil based, and the colors are water based. Then let them put in drops of dish detergent (Dawn works great for this.) It breaks down the oil, and the colors expand all over the dish as if they're alive. They can keep doing this until it's all one disgusting greyish color, and that can take a while.
Goop, which is just cornstarch and water, is fun for them because it squeezes into a dry ball and then "melts" back into a liquid as soon as they release the pressure. Let them add food coloring, and it makes it even more fun. Plus, they get to walk around with colored hands for days, which reminds them to tell everyone about how they got all those stains.
