Bottle bottom turtles - too cute! Just need a turtle shape and the bottom of a bottle! (for a big turtle) or you can use smaller bottles for smaller turtles. Make it fun and let the kids decorate their turtles
Bottle Bank via Martha Stewart: transform a used plastic bottle into a cute piggy bank. Could use same idea to made little containers for kids to store their collections too (use bottle with wider opening and don't make slot)
How to make a Sub-Irrigated Planter. I'm not sure I would use a two liter plastic bottle though. Over time I would fear the plastic breaking down enough to poison the plant. Perhaps a wine bottle instead.
DIY hanging planters - (the link is for purchasing) - this is easily a DIY project - made with two liter soda bottles painted and holes punctured and threaded with fishing line, or even jump rings through the holes and decorative chain to hang with.