If you don't volunteer now, I suggest trying that with some of the extra time. Tutor for free, join the board of a local non-profit doing something you believe in (they would love the MBA aspect), help build a house for Habitat, whatever. You will probably enjoy the connections that brings, and it may lead to work you can do for profit. Either way, I don't think you will regret giving back.
If you already do things like that, I'd probably look at something you would choose as a hobby when deciding what to make money on. For example, MMM likes to fix up houses, and he makes money on that. The work itself is enjoyable to him and is something he wants to do with his time. What do you enjoy? If you enjoy writing, try writing for money. If you make nothing it was still doing something you enjoy, so the time wasn't wasted.