Oh boy, San Antonio is awesome! I went there a few years back to visit my grandparents that live there, and I had a blast! I do advise to bring some sunscreen, though. I don't know what the weather is like in San Antonio at the moment, but when I went there, it was really hot. It was as hot as a September heat wave in southern California.
On the other hand, there's a lot you can do there. For one, you could visit the Alamo, as S said. It's really pretty on the inside. I believe there is a mission in San Antonio (besides the Alamo) that you can visit as well?
My personal favorite part of San Antonio is the riverwalk. When my family went to San Antonio, we ate dinner on a boat in the river. It was really good food! Course, you could eat at one of the restaurants on the riverwalk, too. I don't know if it's more expensive to eat on the boats than it is to eat in the restaurants.
But yeah, I don't remember that much, but I'm positive there are museums in San Antonio that you can visit. Anyways, have fun on your trip!