What do you bring to the track with you for a day of racing?

Everyone there has been in the same boat. They need something they do to have. Most will gladly loan/sell you something if needed.
Yup. Bring common failure parts that are reasonable replaceable and portable. Other than that I'm either done for the day/weekend or better find somebody with a better parts stockpile than me. Track+car folks are generally helpful-one of the reasons I've always enjoyed the track.