Sounds like a great trip!
Most of my recommendations are on the Houston thread, but I must insist you try my favorite Tex-Mex place,
www.spanishvillagerestaurant.com! It is not far from downtown, and a lot of people come from downtown for lunch.
Also one of my favorites -
www.thisisithouston.com , right on the edge of downtown. Heaps of food! Check out the menus for the day on the website as they vary.
Another is
http://www.nikonikos.com/contact.htm for greek, very lively bustling place in the heart of the always vibrant Montrose area.
I have not been to Thelmas or the Magnolia, but there is an excellent Houston website with "real people"/customer reviews for every restaurant in town. Please check it out. You can sort by type of food, expense, area etc...
http://www.b4-u-eat.com. For example, read the info about the Lankford Grocery, reputed to have the best burger in town (also close to downtown)
http://www.b4-u-eat.com/houston/restaurants/reviews/rsv1965.asp Hope this helps!
Oh, I forgot! The PIGSTAND! On Washington avenue, straight shot from downtown. Excellent for breakfast or lunch, one of the originals. They shot scenes for "Evening Star" with Shirley McClaine there. In the book, she would always go there for 2 0r 3 pig sandwiches after visiting her grandson in prison. You just HAVE to go have one meal there!