One of the best ways to recycle water and soft drinks plastic bottles is using them to build a greenhouse. Many members of the garden can be involved in the process: collecting bottles, assembling them, making the structure that will hold them, etc
Easy step-by-step instructions on how to build a bottle greenhouse
Other useful information from Greenside Up
It can also be fun, like the members of Serenity Gardens show in the pictures:

Side panels and doors made. Just the roof missing.
Side panels and doors made. Just the roof missing.

A line of assembled bottles is ready to be added to a side panel
A line of assembled bottles is ready to be added to a side panel