I found lots of lovely ideas on DIY All Things on Facebook this morning (link to their blog) and it got me looking all around for more fun ideas!
I also found two wonderful ideas on DIYnCrafts blog. I think if you use just small child water bottles (empty of course) with glow sticks you could float them in the pool or bathtub! The other is a Privacy Curtain made from the bottoms of 2 liter or 20oz bottles. They could be painted different colors as well.
And the last one I found on pinterest. I just searched for Plastic Bottle Crafts and these came up. This one is just for plastic bottle bird houses or feeding stations. You could fill them with seeds and place a little landing out side the front for them. :)
I think that's about it for now. If you have other ideas, comment them!