*Cough* Freeride trials :wub:
We've done an indoor competition where everything was seriously rock solid. you dont need to nail the pallets, our secret was to get those adjustable car straps for about £3 each and just strap a stack of 4 etc together. This creates a very strong structure without damaging the pallets AND you can re-use again.
Also if you are using blue pallets, make sure you keep an eye on the plastic bits at the end. They bend quite quickly and when someone rides on it harshly.... *crack!*
As for the tractor tyres, obviously you can have one upright but if you lie 1 down and put a pallet on top... voila! a bouncy castle (Y):D
PM me mate if you want a few more ideas =8¬). Hope this helps