I'm from San Marcos and fly fish the river once or twice a week. The advise of the flies here is good. I'd add size 4-8 Clousers as a go to as well. Lately I've been targeting Sunfish with small topwater stuff size 8-12 and been having a blast. Last week in a 2 hours span I had to have caught 70+ with a hand full of small bass mixed in.Foam Dragon - Bream Fly
I've been tying these in various sizes and colors, with great results.
My question would be where are you putting in and taking out? If your putting in just downriver from town and going ten miles on a weekend and your objective is fly fishing you need to rethink. A large section of that (between 101 and Don's Fish camp) will be completely inundated with drunk tubers. Certainly a lot of nice eye candy, but it will render the river unfishable with a fly rod as there will always be someone behind you.
If you want to do a 10ish mile float with excellent fishing and no tubers I'd do the float between Staples and Fentress. From what I understand this is the section that guides usually do their trips. I really like this section of river. No damns to portage (except at the start) and less big slow sections. If you fish it hard, It will take a full day so start early and a rope will be helpful for the portage over the Staples damn.
You can also do Martidale to Staples, it takes about 6 hours if fished hard. I like the first half the second half is mostly the lake above the Staples dam where it's harder to find the fish.
Watch the videos on this website and look around it to gleam some knowledge:Local Guide More
Local Club Website:Local Club Website
Shuttle service if needed: T&G Canoe Shady Grove
Also, If you can join the texaskayakfisherman local board San Marcos/New Braunfels - you can find trip reports and what not.