Each team must create a mini-raft for a "Bear Grylls" doll and then send it down a local stream. The raft that travels the furtherest or arrives at a chosen destination the quickest wins.
Give teams a pile of random materials (or alternatively have them comb the area for stuff) to build a raft with e.g. empty tins, polystyrene, string, rope, bamboo, wood, cardboard, plastic sheets etc
