I got some great advice on Mamasource before we went on our San Antonio trip last May. Try to find time to visit the Children's Museum (downtown) and the San Antonio Zoo. Both are a lot of fun. The zoo has a train and a little beach area where kids can play in some shallow water. to cool off.
If you are interested in riding the big rides at Sea World, you might check ahead that morning before going. It seems every time we have gone, at least one of the roller coasters has been broken for the day. Very disappointing when they only have a couple to choose from.
It's pretty fun to spend a few extra dollars to buy food to feed the sea lions.
Check the feeding times for the different animals when you first get there so that you can plan your day around that.
Be sure to check around for coupons to all of the activities I mentioned, including Sea World. Last year, they had an unpublicized deal where you could bring a pepsi can in to get an adult admission for the kid price. I called Sea World directly for that deal.
There is some site online called something like San Antonio SAVE where you can get coupons.