Back to the early '90s for another one of my unfinished sentences:
The most constructive and profitable habit one could pursue on one's own time is...
...crock pot cookery. (Dan L.)
...sleeping. You learn a lot of what your true feelings are when you dream. (Kristen H.)
...reading. Yeah, that's it. Reading... (Rachel N.)
...interrogating the people you meet. The guy who sits next to you on the bus has a more compelliing story than the highest work of Dickens. Your job is to find it. (Michael M.)