How can I air-condition a garage without modifying it?
Unless you can insulate, it will cost a ton of money.  I get that it's a rental, but maybe the landlord won't mind if you insulate the garage.  Having done that, get one of these AC units that only require you to make a 6" hole through the wall:



Talk to your landlord, they may not mind these slight mods, especially if you pay for the insulation. 

However, if you really can't modify anything, another option is to build a smaller insulated shack in the garage, and AC that.  Or, build a temporary wall to isolate part of the garage as an insulated and AC'ed space.