We obviously need a "Foodie Houston - Where did you eat today" board.
Les Givral's Sandwich & Café does have great Bahn Mi's. Great suggestion.
Flying Saucer is fun and close to downtown.
Venturing further from the downtown area:
Hey Merchant on Washington that has a wonderful beer selection and really good food. Great burger and a fried crawfish sandwich that is really good.
There is also a place on Washington called Max's Wine Dive with a good wine selection and upscale 'comfort food'.
Gatlin's BBQ is considered some of the best in Houston. Only open nights till 7pm close Mondays.
For Americanized Pizza, I like Starr http://starpizza.net/
and Luigi's http://www.luigipizzamidtown.com/
I wish I could recommend a Chicken Fried Steak place to you but the ones I like are too far from the downtown area to make even a cab ride feasible. But I think everyone who visits Texas should try one.
Hope you have a good time.