If you're working for the gov't and/or a large corporation, they'll do all this for you, they've got this down to a science. They keep the cars in covered warehouses, and drive them around every month or so, change the oil, charge the batteries, etc.

Otherwise...this is surely not an all-inclusive list, but .... you definitely want to keep rodents at bay, because the last thing you want to come back to is to find your engine and or exhaust or HVAC full of mice and/or their cache f seeds and/or find your leather upholstery eaten by hungry mice. Yes, they will nest INSIDE the motor and INSIDE the HVAC system. And shredded mice do not smell good. Plug EVERY possible point of entry, stuff steel wool up the tailpipe and seal all the air intakes, put lots of moth balls around the interior, the supposedly discourage the mice. Also, you don't want to let the tires flat spot, so put it on jackstands or steel wheels. Take the battery out, replace it when you return. Fill the gas tank, then drain it and refill with fresh gasoline when you return. Also do a major service when you return, changing all fluids. I'm sure I've forgotten something, though.

It sill probably makes more (financial) sense to sell the car, invest the proceeds, and buy another one when you return, IMO.

-Punxsutawney Phil