Downtown San Antonio is loads of fun. Be an early arriver to everything, and you should be fine. Do not know where you are staying, but traffic is something you will need to consider when traveling to Sea World. Leave about 40 minutes before the park opens and plan to wait. If you get in the line early to park, do not pay for premium parking.
At Sea World, as you enter, go straight to your left as you pass through the gates to see and feed the dolphins. It is less crowded early in the morning, and the dolphins are hungrier then. Pay close attention to your location and where the shows are located. Arrive at least 15 minutes before each show, especially the Bulugas and dolphins. Take some extra t-shirts, towels. If you sit up front in the splash zones, the water can be pretty cold.
The Riverwalk boat cruises are the most cost reasonable of all things to do downtown. The prices are just awesome. I think the best time to stand in line to do this is when everyone else is eating.
On the way to or from San Antonio, and if you have time, Austin, New Brunsfels are all great stops. The trolly, downtown Austin, is free, and it will take you to see things that you didn't know existed.
Hope you have lots of fun. We did.