No one said first aid kit. I keep it in the trunk but I always have it. I've seen guys almost bleed to death on the flight light. I also keep a towel back there (better to stop bleeding than tying it off). Also a cell phone and knowing where you are in case you have to call for help.
Aside from that I carry almost everything I need to build a plane in my toolkit. The trick is it stays in the truck. I have a flight line box with the bare essentials. Hope this helps. BEWARE THE PROP!