I did a project in my class where we changed a soda bottle into a lung using two straws, two balloons, a plastic bag and some tape. The only part you would need to do for them is cut a hole in the bottle cap.
Two liter bottles, with a few simple holes can be turned into bird feeders, or can be filled with dryer lint and hung for birds to insulate their nests in winter. They can also be cut in half and used as seed starters or water bowls for outside animals.
Two liter bottles are great craft materials for young children because they are inexpensive and teach kids about the value of reusing household materials. Kids can make scoops, bird feeders and hanging mobiles that catch the wind with two liter bottles.