Using prepared pipe lagging up in half. Beavers build a marble run
18mm piping lagging, duct tape
Before the meeting cut 18mm pipe lagging in half. This will leave you with a semi circular foam length with channel for marble to run. Split the group into 4 or 5 and using other materials found in hut (chairs, table) build a run for the marble to run down. Bonus points if group can do a loop the loop.