Use basic pioneering to construct a ballista and fire bean bags (or balls, screwed up paper etc.)
Bamboo canes, elastic bands, small plastic bowls (drilled with 3 small holes @120 degrees), bean bags.
Cut up inner tubes can also be used as the firing mechanism.
Using the canes and elastic bands (or string if doing lashings) make a triangular pyramid. On one face attach the elastic bands to each corner of the pyramid - attach the other ends to the bowl. Place bean bag in bowl, pull back and fire. We have competitions on accuracy of firing by having buckets as targets. See attached photo.
5 minutes to plan, 20 minutes to make, 5 minutes tinker time, 15 minutes to try them out/ compete!
Possible marking scheme - up to 10 points for each of: design, construction quality, range, accuracy, teamwork