My kids go buggy over this open-ended project: decorating a colony of brightly colored beetles. We usually spread out our bug-making over two afternoons--one to mold the beetles and one to paint them.
To start, mix a cup of plaster of Paris according to the directions on the box. Pour the mix bit by bit into disposable plastic spoons and let them dry overnight (we usually make 20 to 40 at a time and save some for later). The next day, pop the bugs out of the spoons and have your kids paint them with acrylic or tempera paints. An insect guide can offer some fun inspiration for colors and designs. Painting the bugs can take up the better part of a rainy afternoon, but you can also extend this project in other ways. For example, make your beetles more functional by gluing them onto magnets. Or challenge everyone to create an interesting beetle cage from any supplies around your house (toothpicks are a good jumping-off point).