Depends on how you go to the track.
If you go with only your car, and use the same tires, then you can carry various hand tools, cordless drill, spare brake pads/fluid/rotors, air can, foldup chair, umbrella and/or poncho, lunch, camera, clipboard and paper to record vehicle settings for each run, tire gauge, race tape (for headlights), a tarp, shop towels/rags, HELMET (don't forget that), etc. Some people will take out the spare tire for weight reasons, some people will leave it in for weight reasons. IF you carry race-only tires (inside the car, and yes it can be done), then you immediately become space-limited for some of the previously mentioned items.
If you have a friend or wife who will drive another vehicle, you can load all that stuff and more if needed. Bottle jacks work well for swapping out tires instead of having to carry large conventional floor jacks.
Mostly just depends on what between-round maintenance you think you may need to do. Some people will swap out pads/rotors for track-only pieces (mark them appropriately for each side) so you may want to bring a floor jack anyway. Don't forget a torque wrench so you don't over or under-tighten the lug nuts. If you need to limit what you can carry, pick the tools for the jobs you plan to do and only take them. Leave the kitchen sink in the kitchen.
Make sure you clean out all loose stuff from the car, including the glove box and the console, and remove the floor mats. Also, make sure you don't leak anything as that will get you DQ'd real quick.