Here is another promotional product idea: custom plastic bottle caps! The prints were made by Gregg (one experienced technician) and PromoJET (industrial ink jet).
These bottle caps were flame-treated prior to printing. The treatment can be done quite a while before you print if you prefer. In other words, you can flame-treat the entire tray of parts and let them sit for a while. As long as you refrain from touching the surface before printing, they will print with great quality and adhesion. If the parts have been sitting for a while, you may want to blast the parts with some heat (heat gun or oven) just before you print the tray. This will provide even better adhesion than if you did not bother.
About 20-30 large caps or about 50-60 small caps can fit on the inkjet’s tray. No matter what size cap you are printing, it will take under 10 minutes to print a tray of these at the standard resolution of 1440dpi.