O.K. All
I am requesting your help. I will be leaving S.Calif at the end of Sept. and going to Il. Via U.S. interstate 70. (not as hot as going the southern route) There will be two of us my DW and myself. I want a bag to take for unforseen problems. This bag will be in the back seat of a rental car, and I have chosen a plain black and gray day pack which is 2,310 cu.in.

Sorry to say no guns. I think everything else is o.k.
I have a lot of things for it but I don't want to list them so I can get your ideas

other than my "survival tools" I will not be taking tools so we can leave those of the list.

GO

I take my backpack on all my trips. It carries things like a bag full of breakfast/granola bars, some of those single size packets of lemonaid and gatoraid mix, and some bottles of water for the food and water.
Another small bag inside with matches and lighters, etc.
A small first aid kit.
A few small tools, leatherman squirts and the wave bit adapter and bits,
buck 110 knife, compass, binoculers.
AA battery charger (maha c401fs) and a few packs of aa batteries for the camera and such.
change of clothes should I need to get dirty changing a tire.
Pill bottle of quarters, old wallet with some cash.

+1 on the headlamp. Petzl e+Lite is nice, compact, and light.

Also some T.P. and/or Kleenex, baby wipes, Advil, Immodium, Pepto, eating utensils or sporks, Leatherman or similar multi-tool, SAK, duct tape, zip ties, basic compass, flashlight, spare batteries, camera, first aid kit, emergency ponchos, trash bags, bottles of water, snacks, medium-sized cooler.

And oh yeah... most importantly... road maps!

Ha Ha.
FINALLY A MAP IS MENTIONED.

Okay everything said above is awesome and maybe these don't go into a backpack but....

Doritos Corn Chips, best road trip food ever.
Case of bottled water.
ear plugs and eye cover for sleeping.
Ipod.

Suggest carrying under the seat a sharp machete and 24" or larger bolt cutters.

Wildman800 (and others) wrote about a Personal Support Kit to ride in rental cars:

http://forums.equipped.org/ubbthreads.php?ubb=showflat&Number=158083&page=1

Also add sunscreen in case you and DW have to walk anywhere from a stranded vehicle.

While the back seat is good, make sure you get in the habit of taking it with you when you get out of the car; every time.

Important documents such as copies of driver's licenses, credit cards, rental agreement, IDs and so on.

$.02

spare key.

YES YES YES!

Ask the rental company for the spare key to the car. Since keys now have security chips, running to a big box store and making a duplicate will not work.

In 1999 it was $500.00 and more than 24 hours for Budget and Ford to get me a key to my rental. I dropped something on the key and broke the "handle" part off. Guess where the chip is....

I thought all did away with the chip in the key head. Ours "read" the key via hall effect sensors in the ignition switch so a simple copy does work. This system replaced the old resistor in the key system.