Fill the gas tank up with gas and add a stabilizer. This keeps condensation from forming on the inside of the tank on cool mornings.
Air the tires up to their maximum pressure, say 45 or 50 psi, so they won't flatspot as bad.
Use tennis balls or the like and plug the rear exhaust and the air intake so the critters won't have a place to live while you're away.
If it is due for an oil change, don't let it sit with old oil in it. Change the oil now and then you're good to go when you're ready to get it back.
Leave yourself a note on the steering wheel covering everything you've done so that you can correct the items before you place the vehicle back in service.
There might be a few more items, but this should get you through 6 weeks.
Jr