Of course a popular answer here would be HubPages, but would you have a constant Internet connection? I mean, there would always be libraries and coffee shops I suppose.
Anyhow, here are a few other ways to earn a supplemental income:
1) Donate plasma. This is something that you can do as long as you meet certain health criteria. No, you can't actually sell your plasma, you donate it, but most places pay you for your time. You can do this a couple of times a week, which could earn you a couple of hundred bucks a month. Not bad for laying back on a couch and watching TV for an hour each time. As an added bonus you'd be helping save lives!
2) Collect and recycle scrap metal. No, don't go around stealing copper from pipes and air conditioning units, but if you find junk metal, aluminum cans, etc, you can collect these and bring them to a recycling center. You'd be surprised at how easy this is, how quickly you could raise a little bit of money, and how much you'd be helping the environment.
3) Clinical research studies are another way to make money. You have to be careful though as some of these may ask you to take experimental medication, but some of them do pay quite handsomely. If you qualify, and if you can stick around for the entire time of the study (some are just a few days, some last months, others last quite longer), you can make anywhere from a few bucks to thousands of dollars.
Well, these are just a few things I can think of. Good luck, and safe travels!