What can you do with 20 oz plastic bottles?

Make a greenhouse.... https://www.greenoptimistic.com/plastic-bottles-greenhouse/

Make a vegetable garden... https://www.pinterest.com/pin/576108977308834869/

you can use them in your garden to make an irrigation system. Poke holes around the sides of the bottles. Then, bury all but the opening in the ground between plants. Bottles collect water from rain and give garden an extra boost by letting out the water directly to the plants roots.

You don't say what shape they are, if it's a 1 liter or 2liter turn up side down cut off bottom, fill with dirt and Plant something in it. Get a tripod rope and hang. Where the cap was the water will spill out so you don't, have root rot

There's lots of fun ideas from fellow Hometalkers: http://www.hometalk.com/search/posts?filter=water%20bottles Have fun!!!