I can't believe nobody has mentioned the K brand plastic paint!
Seriously latex, the cheapest you can get is probably easiest, roll a coat or two with a short nap, or foam roller and you're done. Sheet vinyl, or window tint is the most durable which shouldn't be a big deal anyway, still pretty easy. Cheap spray paint, almost any kind will do, requires masking, ventilation, etc... and is more of a pain in my opinion.
If you want to get crazy, there is a thread somewhere on shadow boxes, great 3D effect!