For this fun, all you need is a box of starch based, biodegradable packing peanuts. Dip one edge onto a wet sponge and then stick it wherever you like. Squash them, connect them, wad them up as you like.
Our neighborhood boys had a preference for pokemon and starfighters! The sponge keeps the peanuts from getting too wet-- otherwise they dissolve into a sticky goo. Of course, the boys had to try this out, too.
JoAnn Crafts sells a bag of these for about $5, in a rainbow of colors. They are more durable-- don't dissolve as fast in the water, not quite so sticky, and also a lot of fun!