There are a few options that have crossed IRC...

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Gameboy

At least three variants, all with the same 4-arrow button and 4 other buttons (effectively 8 buttons). All have some sort of serial "link", some even infrared. [a hardware FAQ]

* Gameboy Original, 160x144 4-grayscale LCD, Z80 based. There are millions of them around, going rate is now in the sub-$10 range.

* Gameboy Color, 160x144 x 56 colors and some other enhancements. Not nearly as common as the original, and discontinued.

* Gameboy Advance, 244x160 color TFT. 512 color sprite mode, 32K in bitmap mode. Arm7 CPU, going rate on ebay around $60 used. Still available new around the $100 mark. Flat and folding form factors.

* Gameboy DS ??? Has touch/pen pad. No good.

Pros

* Ergonomy. Large buttons and large (though slightly grainy) screens that are good for small kids, are also good for large kids in moving cars.

* Fairly cheap, widely available.

* Intuitive to use and navigate tables/menus with.

* Easy to program?

Cons

* Will it be taken seriously? [TurboXS] thinks so, and is setting precedent.

* New variants might start coming faster and faster, with the old ones discontinued and becoming harder to find. The advance is likely not going to last 10+ years like the original did.

* Entering numbers (table work) may be tedious with a limited number of buttons.

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PDA

Pros

* Fairly common and standardized.

* Usually a relatively large color screen.

Cons

* No buttons.

* Touchscreens suck in a moving car.

* Can be expensive if you don't already own one.

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Java phones

Pros

* Compact.

* People generally take it with them anyway. Many cars are fitted with holders.

* "Everyone" has one, or can get one basically for free.

* Has numeric keypad.

* Easy to program?

Cons

* Not all the same (different button arrangements and screen resolutions)

* Screen and buttons are small. (example: mine has 128x128 color TFT). No room for multiple pretty "gauges". Bargraphs or some other colorful creativity will have to do.