Use a bottle to mix up your own foaming soap mix. Take 1 part regular liquid hand soap, 1 part water, mix gently. You're done. Label bottle "foaming soap refill".
Use them to hold 1-2 balls of yarn or string. You can also put rolls of toilet paper inside a 2L to keep it dry for camping/canoeing/white water rafting.
Use them to make a bank for a child and hold coins.
Several ideas for using pop bottles - yarn holder, toilet paper holder (2L), plastic bag dispenser, funnel, scoop for birdseed, fertilizer, etc. I use my bottle funnels for pouring used oil from my oil catch pan back into oil containers so I can recycle the oil.
BIG projects using plastic bottles - a floating island using 250,000 bottles, a house, doggie armor, solar powered model car, a raft, and more.
Wrap fishing line around it, and you can use it as a fishing kit. Store weights and hooks inside.
Cut the top off. Now use it to hatch fairy shrimp or triops. Or hold a few tadpoles.
Cut the bottle in half the long way (top to bottom) to make an underwater viewer.
Plastic Cap ideas
Use them to mix epoxy. I also use them to mix paint, or pour a little paint into them for touch ups.