A large pile of cardboard tubes, empty boxes, plastic bottles, tubs and a few rolls of sticky tape. Just add marbles.
Our challenge was for each of the groups to make a marble run at least 2 meters long and where the marble would drop into a container at the end. If that's not enough of a challenge, then try to get the marble to do a trick.
In this case getting the marble to bounce off the trampoline into the next run.
There was some debate about whether it was cheating to use the tables as part of the run, but Reduce, Reuse, Recycle. They certainly reduced the materials used and tables are reusable so....