I think you completely cover the paper with an assortment of colors then heavily color overtop with a black crayon, then scratch it off with a coin or some such thing. The first crayon sticks to the paper, the second just coats the first.
I just tried it as I was typing, with a purple crayon instead of black and the edge of a binder clip and it worked.
The homemade playdoh is a good idea, but if you're going to be mixing in the kitchen I'd just as soon make the cookies!
My kids are always content with a bucket of crayons and a stack of paper and if you really want to keep them entertained - give them permanent markers! (just kidding, I don't advise that, but they really love it when they find them.)
They also like to clean windows and sweep. These are things they don't do, so they think it's fun when they are allowed. Granted, they don't do such a hot job, but the dogs will smear up the windows and shed on the floor shortly anyway.
The teachers suggested a fun way to learn spelling words, shaving cream on a cookie sheet. Then the kids draw out the words. I haven't tried that one yet.
If you have paper grocery bags, they could make their own monster costumes.
And if all else fails, like wonderwoman said, if I threaten with cleaning their rooms it's amazing what they can find to entertain!