I lived in San Antonio for five years as a kid, and went back about three years ago for some Vacation. You absolutely have to visit the Riverwalk. There's a great places to eat, and places to grab a drink....I went Biergarten in September a few years back they had some Oktoberfest activities going on. You can also do a Riverboat tour for like ten bucks.
Also, one of my favorite places to go to as a kid was Six Flags Fiesta Texas. If you have some time and money and love roller coasters, I'd go have some fun!