all fluids, hubs, ignition system, filters maybe drivebelt
if you are really into it, balance rotate and a tire alignment. greese every hinge and cable. wash clay bar wax. check swivel preload and a thorough assessment of suspension bushings and steering components.
at very least change your oil (Amsoil 10w40) and grease the universal joints.
Poor gas mileage gets you to the best places on earth