I want to play games. Nintendo, sega something on the screen. How?

I'm looking into getting an alpine double din headunit with video input. How can I use this to play games in the car. Reason is I end up sitting in my car a lot waiting for people. I want something I can instantly turn on/off and I have a usb nes controller. How can do this without a full onboard computer?
And are there any systems that have all the games built in? I would like to keep it the least cluttered as possible.

i should warn you-- this forums primary focus is in-car computing, so our answers are biased in the direct area you don't want to go. maybe look into one of those '10 million games in one' boxes that plug into a tv? really not sure-- i would just install a xbox or playstation in the car...

^^^ I agree with the above. But if you don't want to install, I would buy an iPad 2. Best portable gaming platform, IMHO.