You could do a bird feeder, wind spinner and more!
Here's a link with lots of ideas:
Here are some ideas from right here on Hometalk: https://www.hometalk.com/search/posts?filter=2%20liter%20bottles
There are some good ideas here: http://www.bystephanielynn.com/2012/04/25-things-to-do-with-empty-plastic-bottles-water-soda-bottle-crafts-saturday-inspiration-ideas.html
You can cut off the top and use it for a funnel. Then use the bottom for a planter. Add rocks and soil and plant seeds. You could also decorate it with decoupage.