Flotation for packs, bundles, luggage, etc.
Sterilize water? (I've heard of successful experiments with clear plastic and UV from sunlight, but can't speak to their reliability.)
Wasp/hornet trap. (Very effective.)
Trail marker. (In wild country, man-made objects stick out like a sore thumb.)
Weight (water or sand) to add "spring" to tarp guy lines in gusty winds.