You don't have to be a professional food photographer to take good photos of food while on the road. It's actually quite simple.
Time: The best time to take photos of food is sometime in the afternoon after 2pm. You'll beat the dinner crowd, and you won't be annoying the entire restaurant when you pull out your camera.
Location: Find a nice place next to a window in the front of the restaurant. Why the window? The natural light coming in from a window acts as a large softbox, producing a soft, diffuse natural light that is the most desirable light for food photography. You generally want to avoid shooting food in artificial light (e.g. flash, interior lights, etc.).
Set Up: Very simple. Place the subject so that the light is coming in from your side or behind you. This this way you'll avoid any areas of the image that are too dark or light.
Exposure: Any Auto Mode setting should do the trick. Personally, have my metering mode set to spot meter and I meter for the light that best captures the food. (See how to properly expose light here).
Aperture: Use a low aperture. F2.8 or less is ideal. The shallow depth of field will really highlight the food in your image.
If you're feeling adventurous: From the diagram above, you can see the set up is relatively easy. If you want to lessen the shadows furthest from the light source, simply find a white reflective item. White paper or any semi-shiny surface works best. Hold it so that it catches some of the natural light from the window and bounces it back into the areas of shadow for a softer toned image.