L.T.
What about having some crafts on hand? You could pick up cardboard boxes at a craft store and let them decorate as they wish - markers, stickers, foam stickers, glue-on foam shapes. Or have them make collages out of construction paper, sequins, craft feathers, foam mosaic tiles, pompoms, etc. Crayola Color Wonders products are great for no-mess fun.
Or just have some balls, blocks, cars, puzzles on hand.