Sounds like a great trip! I'll add a few things for you to look into for San Antonio and Austin.

San Antonio

• Pearl Brewery: No longer an actual brewery, but a neat collection of restaurants and shops. I really enoy their Farmer's Market on Saturday and Sunday mornings.
• King William Fair: My favorite Fiesta event, especially the parade.
• Bar America: Dive bar with a kickass jukebox.
• McNay Art Museum: Favorite museum in San Antonio. The grounds alone are incredibly beautiful. Thursday nights are free.
• The Fruteria: Love this place for dinner and drinks. I cannot recommend the Puerco en mole blanco enough. Yum!
• The Esquire Tavern: Swanky bar on the Riverwalk that has live music on Tuesday nights.
• The Menger Bar: If it's good enough for Teddy Roosevelt, it's good enough for me. Besides, the Menger is the "Most Haunted Hotel in Texas"

Austin

• Zilker Park: Spend a sunny day at Zilker and Barton Springs. There is a nice food truck park nearby, the Picnic, that is BYOB. I recommend Turf 'n Surf.
• Butler Trail: Try the 5K loop for a nice jog or walk. Trail goes around Town Lake/Lady Bird Lake.
• Saxon Pub, Continental Club: Good places to catch live music
• White Horse: Honky tonk + hipsters. Dancing lessons. Don't mind the smell. Younger crowd than Broken Spoke, Donn's Depot (which are still worth checking out)
• South Congress: Shops, restaurants, people watching. Continental Club is located along this strip.
• In addition to 6th street, both Rainey Street and the East Side (ex: Shangri-La) have night life worth checking out.

Honestly, I would skip the zoo in Austin. If you are set on visiting a zoo, I'd pick the one in San Antonio.