Ideas I've had so far are...
Designing items for use with cos-play. I don't personally participate, but the people who do are required to have x-percentage of their costume "made" and not bought from a store. Some items like these are already on thingiverse (random example: http://www.thingiverse.com/thing:26927) and could be printed off very easily and solid for a decent profit. Especially if they're wanting a finished look using the ABS/acetone mix it could be a fantastic piece.
Another one I did for a friend and has already got a whole ****load of people wanting one for their friends/family was as simple as using one of the open source designs for a 'coin-op' beer bottle opener. Using Google Sketch Up it takes a few seconds/minutes to change out someones initials on the beer bottle opener, and, ask for their birthday. I keep a jar full of pennies, went through it and found one that was on my buddies birthday, and inserted it with the date showing. Simple way to personalize an item, and can turn a 50 cent piece into a 3-5 dollar buy for someone. I intend to design one of my own just for the sake of reducing how much plastic it uses and any issue of me selling them as well as maybe doing an inlay.
Doing a 3d model of someone or a couple is another option, the software to do it with pictures is free from AutoDesk, however not having used it before I don't know how massive a pain in the butt it is or is not. Those could easily bring in 20-50, maybe more if there was a lot of time put into the render.
The other option is to start building 3d Printer 'kits' in the sense that you provide all the printed parts necessary, and they buy the other parts elsewhere. since a lot of people don't have one of these yet, and seem to like the RepRap idea, etc. Download the .stls for the parts needed for a Prusa Mendel or whatever it is you want and go.