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?)