How long is your trip? I can't see doing more than a couple of days in SA.
You could add Schlitterbahn in New Braunfels to your trip. If you go in April or May the dates are very limited on their operating calendar. The rides are fed by water from the Comal River and it is very cold. So, spring is a tough one for Schlitterbahn. We have been in May for school field trips and the kids were turning blue early in the day and later in the evening. On the upside, the crowds are non-existent in April and May in the waterpark.
Gruene, Texas is a fun day trip. It's a few minutes from New Braunfels and has a cool main street area with restaurants, shops, wineries and bed and breakfasts. The Gristmill is a favorite restaurant that has been around forever. There is also Gruene Hall, the oldest dance hall in Texas, with live music and dancing.