Provide Beavers with a ground-sheet and tell them to build a den
Ground Sheet, buckets for water
Explain to Beavers that the challenge is to build a den and they only have a ground-sheet to use. At the end they will test it with as many willing volunteers as there are to see how waterproof they are. Advise Beavers to bring a waterproof! Tell them that they should try and also camouflage it. You could make up a story around the challenge to add authenticity. Ideal in a wooded area.