Well, if your flashy enough you could be a martial arts movie star I suppose, usually you don't have to be able to act. You could own a large chain of McDojo's guaranteeing a bunch of rather pathetic types blackbelts for learning very little and doing very little if the price is right. Going along with the McDojo idea, you could make it a sort of a daycare, and even pick up the kiddies in a van and give the parents a break if the money is right (This last one would be like hell for me, but the money might be good) To be less sarcastic, you could run a very good school and make good money as well.
Bodyguards sometimes make good money, of course you will need to know something about guns, which really are modern day martial arts weapons anyway, and other things like putting your body in front of your client, and really, getting a long with your client is the most important thing for a bodyguard according to a former bodyguard I once asked, he also said hours are long ...
You could send mail order blackbelts and instructor certificates I suppose, I couldn't live with myself selling such things...