Be sure to check out Acenar. It's a fabulous Mexican restaurant right on the River Walk!! My mouth is watering just thinking about it!!
We love Acenar! They have the best chicken mole! DD(8) isn't a fan though, since it is more traditional or even upscale Mexican food than TexMex. We always make her go with us once when we go to San Antonio even though she doesn't like it.
OP, really if you are just looking for Tex Mex food, you don't even have to plan ahead. Just go down and explore the river area. There are restaurants everywhere, and many of the menus are posted at the entrance. We usually spend one night where we don't have plans and just walk and pick whatever is appealling as we walk by. We've done Rita's, the Original, and Casa Rio and liked them all. I think the prices are reasonable at all of those.
I will say that last December, our BIL wanted to go to the Iron Cactus with a family group, and it was not good. Poor service and nobody cared much for the food. My chicken was overcooked rubber.
The irish pub is a lot of fun.
If you do decide you are looking for something a little fancy and pricier, you can eat at the top of the Tower of Americas at Chart House. Chart House is a chain that you find other places across the country, but the food is good and the view is terrific. It is pricey and you will need a reservation for that though. If you don't eat there, but still want to go up to the top of the tower, they do sell tickets to an observation deck for $11 each, I think that also includes some sort of 4-D theater ride, but we have not done that.
Mi Tierra that someone mentioned is actually down in the market square area instead of right on the River Walk. That can be kind of a fun area if you want to explore the market. They often have live music going too. We have been to Mi Tierra and enjoyed it, but it is very often packed with long waits as it is the historical well known restaurant down there. There is another restaurant just down from it called La Margarita and we really like that one. We have always had good food and drinks there. I like their chicken mole (which is a theme for me in San Antonio because there is no place around our home that has a decent chicken mole!)
I would get a hotel on the Riverwalk if you can. There are a bunch to choose from. The prices will depend on when you are going and what else is happening in town on those dates. From a hotel on the Riverwalk, you can walk to a ton of restaurants, pubs, shops, the Alamo, Tower of the Americas, La Villita (a little arts district by the River walk) and more. Market Square is a bit of a hike, but we walk it too.
Also, they do carriage rides around the Alamo area at night. We had fun one where the lady told us about all the haunted bulldings and ghosts in the area. We also had another one where the guy barely talked (which that night was exactly what he wanted- he might have picked up on that when DD told asked the please tell no ghost stories.

) . There are ton of carriages and drivers to choose from, so if that matters to you and you want to hear about the history or ghost stories, I would ask the driver before you pick one if they have that kind of information to tell you about during the tour.
Other things to do in San Antonio include Sea World or Fiesta Texas.
About 40 minutes away from the Riverwalk/downton in San Marcos is a huge outdoor outlet mall
About 30 minutes from the Riverwalk/downtown are the Natural Bridge Caverns and the separate Natural Bridge Wildlife Park. We really enjoyed them. The Cavern tour is fun and the caverns are beautiful. The wildlife park is a drive though park where you can buy food and feed the animals as you go through. They also have a little area that has some other animals, a fast food type cafe, and a gift shop. We enjoyed that too. Watch out for the ostriches though. If you have animal food, they will totally stick their heads in your vehicle and into your lap to try to steal the whole bag. DD(8) loves to tell everyone about the ostrich that attacked Mom. I thought I would never get that things head out of the car! Next time I see one coming, I'm rolling up the window before it gets there!!!