Of course, it depends upon how far you are traveling. Here are some of the things you might want to consider:
Wash & Detailing stuff
Car Duster
Swifter Gloves or pads for dusting engine compartment
Tonneau Cover
Car Cover
Rain-X
Fix a flat
Spare Fuses (one of each)
Lead Hammer
Tool box
Knife and window scraper
Scissor jack (necessary if you need to get under the car)
Fan belt(s)
Wipers
Can of radiator stop leak
Duct tape
Digital voltmeter
Spare hose clamps and a foot of fuel line
spare coil
spare fuel pump
a couple cans of
oil
tube of gasket maker/sealer
tire key
GPS
two or three rags
Mechanix gloves and some Goop Hand Cleaner
List with part numbers for radiator hoses, heater hose, distributor cap, rotor, spark plugs, fuel filter, ignition module, fan and water pump belts.
Personal or other:
EARPLUGS
Duffle Bag for personal items (razor, shave cream, prescriptions, etc)
Clothes
Small pkg of band aids
Sun block
Lip balm
Sunglasses
Hat (maybe one for impromptu passenger too)
Jacket
Driving Gloves
Camera and extra batteries
Cell Phone
Cell Phone Charger
Bag Chair
AAA card
Some bottles of water
Helmet and drivers suit/shoes if going racing
Towel
Compact umbrella for walking around if it rains
Large golf umbrella which will cover almost the entire cockpit during unexpected downpours
Large plastic garbage bags with holes cut for head and arms or clear throwaway “jacket in a bag”
Inexpensive clear plastic drip cloths, held on with a little duct or masking tape, make great impromptu water proof car covers
Plus some new springs to hold up all the stuff in your trunk
