As said previously this has been done to death before. However I personally would avoid the charity flight situation. The reason being, I expect a general member of the public who won a flight would in a sense be putting their trust in a stranger ( like you do when you fly a commercial airline), and would probably be expecting a level of expertise and / or service level which doesn't exist. I don't think it would be fair to expect every winner to check out their pilots level of expertise and experience. Nor do I think it fair to expect them to know what questions to ask.
At least when you take a friend etc they know pretty much what they are getting and the associated risks....