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As a native Huntsvillian I can tell you that the above recommendations are good ones (and making me homesick and hungry), but a few key Huntsville experiences are missing:

A classic Huntsville meat-and-three, with walls covered in photos of historic Huntsville and regular patrons who probably remember when it looked like that: Mullins Drive In. Note: not an actual drive-in, but probably was at some point. Go for the catfish, save room for pie.

If you want more catfish, or other fried fish between bread, Po' Boy Factory nearby is a great choice. Both of these places are in Five Points, a neighborhood filled with cute homes and a few little stores. Also close to the more stately Twickenham area.

A correction to the above: Gibson's BBQ with its 4 locations is bad. Big Bob Gibson's (in Madison) is legendary and is the place with the aforementioned white sauce. A fantastic texas-style brisket-bbq-making place also in Madison is Chuckwagon BBQ.

Huntsville may be southern, but it really is a mix of people from all over, including a large German population who have been in town since the 50s (and some who came with the original group of rocket scientists)--German restaurants around town are worth checking out. Also several good Indian buffets.

You can also get a lot of good Mexican and Central American food. A favorite is La Michoacana, which also has a little grocery store. If you want something more Americanized, Little Rosie's is a local favorite.

Hope you have a great time in Huntsville!