Answer:
I'm a small ring shaped fried cake, often with bavarian cream or fruit filling, and sometimes covered in icing and sprinkles.
People want it, and when they have it, they use it by giving it
What can get you there in eco style, pushed by your legs mile after mile?
You fill it and it empties, A metaphor for plenty, Depending on what half you see, It's either full or empty
What object has keys that open no locks, space but no room, and you can enter but not go in?
What do you purposefully put lots of in and on your body, but run away from when you encounter it outside?