I've left cars up there for weeks many times, and never had a problem. This year I'm adding the car cover because the sap dripping from the trees and baking on the paint for weeks is really hard to get off, plus my vehicle this year is black. Not the best for parking in the sun with windows rolled up.
Clean it out - leave nothing smelly in it. That should be good enough.
I found a $32 car cover on ebay and it actually is pretty decent. Not something that will last forever, but for Portal use or other desert parking, it'll do.