Teams of Scouts purchase items from the "Scrap heap" (using monopoly money) then plan and build balloon or rubber band powered vehicles.
Monopoly money (or print your own currency),
Scrap heap (assorted metal/plastic lids from jars, caps from bottles, dowels, skewers, straws, empty plastic drinks bottles, cardboard, rubber bands, balloons, blu-tac (other brands are available!), tape, old CDs, assorted rubber washers, cardoard tubes from kitchen rolls, drawing pins, you could even have printed instruction sheets with a hefty price tag for those teams who can't be bothered to design their own! - just search online - there are lots of ideas available).
A course of a set distance.
Tape, scissors, glue, paints.
Split Troop into teams of 2 to 3 members (or you could just use Patrols if you're sure certain members won't just sit back and let others do all the planning/work).
Give each team the same amount of money.
Allow them a short time to discuss their project and make a plan then purchase the necessary items from the scrap heap (have a set price list ready for your scrap so it is fair to all teams).
Give them a set time to build and decorate their project.
Test each vehicle on a course of a set distance. Points can be awarded for speed, distance, how much money they have left unused, how cool they look, team spirit etc.
Allow a second design session to correct any faults or make general improvements.
Test each vehicle again using the same criteria and announce the winners.