Answer:
What kind of nut is empty at the center and has no shell.
Upon me you can tread, though softly under cover. And I will take you places, that you have yet to discover. I'm high, and I'm low, though flat in the middle. And though a joy to the children, adults think of me little.
What is full of holes but still holds water
When I get closer my tail grows longer, but when I go away my tail leads the way.
I turn around once. What is out will not get in. I turn around again. What is in will not get out.
What do you purposefully put lots of in and on your body, but run away from when you encounter it outside?