Fireworks with matches
You need lots of matches. Prepare at least one box for each participant. Other material may be added if you like (paper, glue, ..)
The participants each get the same amount of matches. From that, they have to design any object they like.
When everybody has finished his object, they present them and then set them on fire.
A jury votes upon the "best" object (best design, best show while burning, etc..