Basic things you will want to take autocrossing:
Foaming type glass cleaner and a towel - I like to clean my windshield after I arrive at an event.
Tire pressure gauge - Longacre's digital model is accurate, easy to read and use.
Magnetic Numbers -
TrackSpec made me a set to match my car and fit on the steel rear quarter panel as the doors are aluminum.
License plate cover - I'm not a fan of my license plate showing up in pictures on the internet.
Tire Pump - Get one that stops when it gets to the desired pressure. I found this one at Target and it is accurate and cheap. Similar rebranded models are sold on Amazon.
Folding chair - Unless you want to be anti social and sit in your car by yourself when you aren't working or running.

Rubbermaid Roughneck Box:
The rough measurements are 19.5" x 15 "x 13.5".
It fits very well in the trunk and is weatherproof.

Battery Tender:

If you do not drive your car very often your battery will last longer with one of these. 
Modern cars do tend to drain the battery when left to sit for a more than a couple weeks.