This is more of a place than a cheat, but go to Area 51 in the middle of the map.

Enable the no cops cheat if you have not beat the game.

Do not fly there due to rockets guarding the base.

Look around the base on the outside for a fence that can be broken.

Break the fence to get inside the base.

There is a Hummer, an Army Transporter and a Helicopter, but be sure and blow up the rockets before flying the Helicopter.