Just curious - are the kids boys? I've noticed boys and girls seem to play very differently at that age, difficult to get boys to sit still.

I have a playdoh and crayon eater here. How about stickers? Mine likes stickers.
Maybe finger painting with whipped cream? Then it doesn't matter too much if all the do is eat it.

To be honest, I haven't found that my 20month old boy is able to sit still for that long. But I agree with Kel, your best bet is with stuff that's tactile - maybe a walk outside to collect things like leaves and sticks and then stickytape them to paper?