Edit: Wooops, the title dont sound totally right.
Should be. "What can you do with others or alone that is free or cost next to nothing"
Inspired from a book that had a section saying that there are things you can do alone that are free or cost something, and there are things you can do with others that are free or cost something.
I will start with some obvious ones..
Walking - park or other place
masturbating (not forbidden to say is it?).