Good suggestions from everyone. I would add a few thing (some suspiciously like tools -sorry) based on my experience with rental cars.

An adapter for the 12V Power outlets so I can plug more stuff in - Generally I carry along a setup for plugging in three devices. Add a small inverter if you need 110VAC for something but I try to avoid that if traveling by myself.

Ipod + 12V adapter w/FM transmitter - gotta have good tunes,even in the middle of nowhere!

12V adapters for my other electronic "toys" - Cell phone, hand held video players/games (as appropriate).

Garmin Nuvi (and bring a Map!)

A small collection of auto fuses (smaller than Dougs PSK) makes a big difference. Odds are you won't need them but the impact when you do can be big. I always get the same car type so I know what fuses to carry.

+1 on the multi-tool (my choice would be Leatherman charge) and I would also add in a couple of small/midsize adjustable wrenches and a couple small / midsized vice grips. Priceless not just for the car but along the way/in the hotels/campgrounds.

7" angled forceps - great for grabbing the silly thing that just fell down into a hard to reach spot in the car or elsewhere.

Camera + accessories, always something photo worthy on a trip. (My camera is digital so "accessories) includes a laptop for looking at the pictures).

Various methods for securing things (zip ties, velcro, tape...).

A couple (Ok I drag along 4) small flashing red LEDs (meant for hikers/bikers) for signaling - most the rentals I get don't include flares/warning triangles.

I won't list the general set of tools I usually drag along other than to point out that there are several nice tool rollup kits that take up very little space and a few well thought out tool choices can help a lot, especially in a rental car on a longish trip.

- Eric (tool addict?)

Not as recently as 2005 I know for sure, because I own one.

The keys are VERY expensive, even when you have all of the codes and are at the dealership.

Not to mention a tad large.

Thanks everbody this is getting good, when I have my bag made up I will list the contents. Please don,t stop, I would like to get some more recommendations.

I have always found cash to be a nice thing to carry while traveling...it clarifies things.

I would add:

- (spare) sunglasses
- something that will protect your head against sun rays - baseball cap, bandanna, ...
- something for handwashing - antibacterial wipes or hand sanitizing liquid/gel
- (binoculars)
- cutlery (fork, spoon)
- small fire extinguisher
- photo of your car from different angles with readable license plate - if someone steals it (which I don´t wish you at all), you will be able to show police how it looks like instead of just describing it

Sorry if I repeated anything already mentioned.

Lots of good suggestions here but the most important ones IMO.
Duct Tape.
Pocket knife.
1st. aid Kit.
Blanket.
Snacks.
Flash light.
Water, lots of it.
And keep your gas tank full. In the west you will often
see signs that will say " Next gas 75 Mi." It's true and even
then when you get to the station it might not be open or the owner
might have gone fishing.
Most often you will find these signs 15mi. on the way out of town.

Buy a couple rolls of Vanilla scented trashbags from the Dollar tree/99 cent store.These are Invaluable as,things can get "rank" quickly!Obtain a few packs of Listerine Breath strips,aside from the obvious use,if you slip a double dose(2 leaves)in your mouth when your feeling drowsy,they WILL reverse, that drowsy feeling quite rapidly!Bon Voyage' Standard Disclaimer for the above stores mentioned

On the spare key. When I was a working guy I'd often go on trips of extended or unknown duration and rely on one or more rental cars wherever I was. The first thing I'd do was have a spare made at a local hardware store. No, it didn't have the chip in it and wouldn't start the car, but it did open the doors very well allowing me to get the factory key from the ignition, or sear or wherever I had left it before accidentaly locking myself out. These spares are very small, and can be carried in your wallet.
Lots of good tips here. Thanks to all.

must be different brands, ours are 2001 and 2004.

Paper towels, knapkins or possible a smallish roll of toilet paper. Not only to wipe up spills, some of the rest stops will have no toilet paper, towels or hand dryers.

Windex or other glass cleaner in a smallish spray bottle. Take some cotton rags with it. This is for the inside of the windshield. Even the normal thin layer of grime which is normal increases the glare especially at night. Clean glass makes driving easier and safer.

I usually throw a small bag of extra clean rags in the trunk with an extra jug of water. That is for face and hand washing. Sometimes it is nice to be able to freshen up a bit after a long drive.

A blanket and pillow. They do not need to be large, just enough to get a bit comfy if you want to nap while the wife drives, or vice versa. Sometimes it really helps on a long drive.

If you take a thermos or two you can have hot coffee or tea.

edit: IO almost forgot. If you are leaving at the end of September be sure to throw a couple of pairs of gloves in. You might appreciate them if you need to change a flat tire or something.