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Long-term parking at the airport is convenient, but it's your most expensive option. "Four-five days" can run you upwards of $60, depending on what time of day you drop the car off and pick it up again.
Meanwhile, parking at all the major Las Vegas hotel-casinos is free. If you don't mind leaving your car in a lot, you can do so without any problem from the hotel (though it might get attention from an alert burglar or car thief).
Another option is to give your car to a valet, so it's parked in a special valet lot. Again, valets don't care how long you leave it there; one valet told us one time that he only starts to wonder about a car when it's covered with dust. It'll cost you a couple bucks for a tip to get your car back.
If you park at, say, Terrible's or Key Largo, a cab to the airport will run you no more than $10 including tip. Or you can grab an airport shuttle for around $10 round trip, not including the driver tip, from anywhere on the Strip.