How many uses for the 2 litre plastic bottle come to mind?

-Bottle
-Funnel
-Cup/bowl
-Improvised waterfilter
-Make shift drip coffee machine. (Cut top part off, invert, add coffee filter, add coffee, poor in hot water. Use hot or warm water depending on the plastic type. Boiling water is bad)

Anything that can be improvised with thin bits of plastic:
- Cordage (thin stips)
- Templates
- Candle holder glass thingy.
- etc..

Dry goods, and of course, water storage. The 2 liter bottles are essentially waterproof containers that are small enough for daily usage. Anything that can get poured down a funnel, such as rice, coffee, or couscous, can be stored in the bottles.

EDIT: I forgot about the self watering and DIY container/tower garden projects that use 2 liter bottles. For someone with very limited space, it offers an alternative to conventional backyard vegetable gardens.

Trap for crawfish and minnows. Crawfish for food and the minnows for bait.

Can catch bugs as well (crickets, bees, wasps).

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.

Two bottles with tops cut off, and insert one inside another. Makes a pretty good wide mouth container, used one for carrying bread baps in a rucksack for a week.

ZIllions. From the top of my head, and in addition to the above:

- compressed air tank (used 2 for DIY snuba device, can be used for improvising the energy source for the pneumatic instrument as well, as it holds up to 150 PSI);

- cut out 4 spoons;

- plants auto watering device;

- plants manual watering can;

- automatic animal feeder (grain/water);

- attach to a tree for a camp hand/face washing station;

- air/water pump (suck or blow);

- water distiller (put green leaves inside);

- plastic is hot-shrinking (apply heat over the plastic ties to tighten them);

- plastic of the bottle and of the cap can be melted to make seals, used as a glue, formed into hard plastic things, used to repair holes;

- the top screw with the cap can be used to seal plastic bags, make tarp attachment points, gather ends of fabric;

- cut out a grain/flour scoop;

- make long air/water pipes (cut ends, stack, shrink/melt joints);

- firestarter (water lens);

- camera "tripod" (1/4x20 screw in the cap, sand/water in the bottle);

- shoes (cut an entrance or flatten and add straps);

- sleeping bag warmer (feel with hot water, wrap in clothes, hug it);

- neck pillow;

- flashlight mount/stand/headband/diffuser;

- fishing bobber, fishing net floats;

- knee/elbow abrasion protection;

- safety glasses;

- smoke mask;

The "Practical Guide to Man-Powered Weapons and Ammunition" has a fairly extensive section on pneumatic launchers, and compressed air potato guns have an impressive array of blow off valve designs. While I'm not advocating armaments, the designs are common to many pneumatic machines.

" cut out a grain/flour scoop;" not just grain or flour. For years we have used scoops made from plastic containers to scoop up dirt an archeo or paleo excavations - nothing else works as well.