To add to this, make sure you have in your glovebox a copy of your OR/CR and delivery receipt, insurance papers, photocopy of your driver's license and other documents you might need. I leave my service manual there too so I don't have to remind myself every PMS.

You need to have an Early Warning Device stowed in your car as well.

Good-to-have's inside the car:
Emergency kit
Trunk organizer
Small trash can

^I agree with keeping things clean. Even if you can't yet do DIY car washing or detailing, just making an effort to keep the inside clean is should be enough.