Adding to the list:

Prop wrench (4 way plug wrenches don't always fit)

Spare prop nuts and washers

Spare glow plugs

Needle nose pliers

First aid kit (to hold all the band-Aids )

Thin CA (as an alternative to band aids )

Battery tester

Buddy cord(s) (if you're an instructor)

Loctite

Prop reamer (if you haven't done this at home while balancing them)

Paper towels & spray (I use simple green) for cleaning the plane at the end of the day

Waterless hand cleaner (I'm not crazy about sliming up the steering wheel on the way home)