Goofy, I know, but one of the reasons I got my CCW was because I couldn't bring myself to leave a firearm unattended in the truck but I didn't want to go without either. I'll usually toss (not literally) a carbine behind the seat if I'm heading into the hills and will leave it in the vehicle if I go stomping around in the woods for a bit. Other than that, any guns stay on my person.
I still think about it now and again and would probably look at the new Kel-Tec SU-16 - polymer stock, folds in half for storage, takes AR mags and is about as cheap (if you can find one) as a Mini-14. More accurate, too, I hear.
If I *were* to leave a pistol in the car, it would be for "carjacking insurance" rather than anything for extended and/or distance engagements. The intended purpose would be to buy me time to get to my "normal" pistol or to make tracks enough to get the long gun into action. I'd probably go for cheap and reliable over anything else, since it'd be a "face gun". Maybe the Kel-Tec P32, a Makarov or a used J-frame .32 Mag or .38 Special.
Or maybe a Thunder Five in .45-70 just to see the look on their face.