If you have a few good nights on wine the silver bladders out of cardboard caskets can be re-inflated and become spacers. The can be deflated easily and pack away very small, and as they wear out you at least enjoy replacing them.
Worst case scenario, wrap them in the tarp and make a raft and drift around in dams or lakes etc.
They also make half decent targets for wasting kids time throwing rocks at them, safely of course.
Cheers