A semi trailer would be a good suggestion, between the intermodal crowd and OTR guys there's plenty of options for people to try their hand at.
The ultimate simple model would be a shipping container...
20, 40, 45, 48, 53, and 57-foot (I think) varieties are out there.
I have seen several former ambulances (box-on-Ford-F350-chassis type) used for private businesses, think "plumbing emergencies our specialty", landscapers, etc.
Tour and charter busses come to mind, as do custom motor homes and RVs. (maybe not too much interest, I'm just typing what the random brain cells fire off)
Back to shipping containers: Many of them have been turned into mobile offices, storage units, and even homes. (The home variant perhaps wouldn't quite lend itself to a recolor, though.)
Sprinter-type delivery vans might be an option.
Be interesting to see what comes up and how they get recolored!