Clean Under Floor Mats
Take the floor mats out and vacuum underneath them. You wouldn't believe the crud that gets trapped under there, even if you don't think those mats ever move while you're driving. You may even need to get out a toothbrush and some carpet cleaner and scrub, especially if you're cleaning up after an NYC winter. Salt melts snow and ice, but it also leaves nasty stains on your carpet that you'll want to get out.
Neglect the trunk liner, if your car has one. Stuff gets trapped under there too -- usually debris from whatever you've been hauling in that trunk. You probably won't need to scrub here (unless you spilled something), but a quick vacuuming will help.