I'm not looking to do anything that I couldn't quickly undo.
What is the original surface? Some kinda' plastic? Polycarbonate? Brushed metal? Painted or anodized metal?
Simplest mods that occur to me at the moment:
- a strip (or strips) of vinyl electrical tape. It comes in many colours. The glue from vinyl tape is easier to remove than duct tape or masking tape.
- a Dymo embossed label with "Russell's Laptop" printed on it.
- a whole bunch of little felt dots, applied in a polka-dot pattern.
- bullet hole decals

- get a high-rez scan of a vintage lunchbox ...

... print it out, spray with a fixative, and use an UHU glue stick to glue it to the case. UHU glue will come off with a damp sponge if you want to change the image. Probably more convincing if it was a really small laptop, like a MacBook Air, or a portable hard drive.
- draw something with an uhu stick, and then apply fake gold leaf (visit a craft supply shop) to the glued areas. (Might not be sufficiently resistant to abrasion)