NYC Foodie, 1 night in Tampa- casual
Agree with Oystercatchers, though it may not be as casual as you are looking for.
I'm going to disagree with GrillSmith (sorry.) Never had a good experience there and none of my friends have either.
There is Crabby Bill's, which is super casual and near the airport on the water. Bahama Breeze-but it's a chain. You can also go to Int'l Plaza and there are a ton of options. I'm on a Grape kick there. What type of food are you looking for and do you have a budget? What do you mean by casual? Toothpick hangin' out of your mouth and belches acceptable kind of casual (Crabby Bills) or laid-back, pretty, you can wear nice jeans but don't look like a hobo kind of casual (Oystercatchers) ?
How about going to Oystercatchers where you can enjoy a tranquil view of the bay? You will be just five minutes from the airport. Also close to the airport is Jimbo's BBQ, La Teresita cuban restaurant (although I'm not sure if it's open Mondays). If you want to dine in a more lively atmosphere GrillSmith is very lively. There are many places but will you have a car? If not you might be limited in where you go.