Items you might want to have in your car all the times:
Duct Tape
Rags & Hand Cleaner
Fire Extinguisher
Flashlight
Motor Oil
Water/Anti-freeze
Fuses or an AGC Circuit Breaker---Linda & Phil
Spare fan belt-even and old one--- DB
Nuts and bolts-various common sizes---sr311det
Toilet Paper

Disposable Tyvec coveralls----DB
Auto Club Card "AAA" ----fairlady66
Spare key in your wallet----DB
Cash. There is no substitute. Holiday weekends can mean empty ATM's, or there just isn't one around.----DB
Bungee cords---DB
Tie wraps,(fix anything) ---DB
bailing wire, couple of feet for hot stuff ---DB
tube of 5 min epoxy ---DB
More from Dave Brsico. I don't carry a spare, usually, but have a plug kit, a fix a flat and for long trips a little 12v compressor/light. Another good thing is a 12x12 piece of 3/4 plywood to put under the jack if you're on a bad surface.
Tools:
Lug wrench
Jack, Scissor or Hydraulic
8mm to 14mm wrenches
Adjustable wrench
Standard and Phillips screwdrivers, pliers, etc.
Spark plug wrench ---DB
Hammer, you never know----DB
For a long road trip you might want to include the following items for your trip. Depending on how well you maintain your car much of the below will be over kill. I put an asterisk by the ones I would take.
Spark Plugs*
Fuel Line / 6ft. of 1/4 inch
Electric Fuel Pump
Fuel Filter*
Jumper Cables*
Fix-a-Flat
Carb floats, SU Type
Carb float gaskets
Leatherman multi-purpose tool*
Test Light, Electrical*
Wire / 14 gauge
Electrical Tape*
Brake Fluid, Dot 4 or whatever you are using
Distributor Cap*
Rotor*
Points-if you still have them
Condenser
Upper and Lower Radiator Hoses
Feeler Gauge (for valves and ignition)
Transmission Fluid (90w) w/pump
Hose Clamps, few different sizes.
Fuel Can, 2 gallons / plastic
Throttle Return Spring (*)****
Sunscreen*
Fist aid kit*
Hand Cleaner
Drinking Water
Cellular Phone*
GPS-they are cheap now*
Contact list for route*
****If one of these springs break-your throttle can hang in the "wide open" position-best to turn engine off making sure you do not lock the steering and slowly brake to a complete stop.
The tools can be placed with smaller parts in a small tool box. You can put some of the larger items in a cloth sack and tuck them into the sides of the trunk.
Another excellent idea is when traveling in a group is to spread the load across the group. That way you will still have the item you need but do not have to carry everything. --Paul
Most importantly-prepare for your trip by visually checking all fluids, belts and hoses. Ensure your tires are in good condition with the proper inflation including your spare. Have fun!!