A Happy Mutant Mobile (HMM) should be all about connecting, collaborating and generally getting people involved. Think mobile maker lab, but also with lots of crowd-gathering appeal.
@chaostic put out many great ideas already, and I won’t even try to top those. Still, I kinda see the HMM with lots of slide-out cabinets, ideally in a fractal way: You slide out two racks from the back, and from each of those, you slide out two more. Don’t worry about loading the racks with actual merchandise (that would foul up the fractal-rack mechanism anyway): Let them instead sport things that can are flat and can be folded out: Touch screen displays, flipboards, free USB-ports for people to drop data into, whiteboards for doodling world domination plans on…
Anyway: Leave room on it for further editing. When you hit the road, you want all those happy mutants you meet to add things to the HMM. Maybe someone will want to laser-etch something onto the hood, or someone else has a spare projector that can be mounted to the roof to project an interactive hopscotch field onto the sidewalk for passing folks?