Pioneer some go karts
This requires quite a bit of preparation and work beforehand, but is something the scouts can get involved with throughout.
Per go-kart, you need:
2 axles - 6 inches diameter, 1m long
4 metal pins to stick out the axles. 1cm diameter. Should have a hole in the end of the pin for R-clips
4 R-clips or similar
Sheet of plywood
Drill a hole at either end of the axle the same diameter as your big metal pins. Pour some glue in them, and whack the pins in them so there's enough space for the plywood wheel and the R-clip.
Cut wheels out of the plywood - I found about 1ft diameter works. Drill a hole in the centre that's just big enough for the pin.
Now for the assembly. Take two pioneering poles and lash them on one axle (Japanese lashing work nicely here). Viola - you have a chariot - someone can pull someone else already.
Now lash the other axle on the end of the poles, so you've got a rectangle. You now have a trailer, or something that can't steer.
Now unlash the lashings on one axle, and bring the two pioneering poles together, lashing them. (So you've now got a triangle, with the axle and two poles). Lash the end of the two poles onto the middle of the unattached axle. Tie some rope to the ends of that axle for steering.
Throughout all of this, you can create more lashing work by including a seat. We ripped apart some school plastic chairs and attached some 2x1 wood horizontally just under them, so the scouts lash the wood to the pioneering poles.
Have races, time trials, obstacle courses - whatever!