Answer:
A disposable item to keep your waste organized.
What flies forever, rests never?
You use it between your head and your toes, the more it works the thinner it grows. What is it?
What object has keys that open no locks, space but no room, and you can enter but not go in?
Mouth up it gets filled, mouth down it gets empty.
I run through hills. I veer around mountains. I leap over rivers. And crawl through the forests. Step out your door to find me.