You must visit the Alamo. Go to the Tower of the Americas for lunch on a clear day. It revolves 360 degrees an hour and it is a great view of the area. Go to the Old Missions and to the Governors Palace. Go to the Riverwalk and take a boat ride. Walk the Riverwalk and stop in a bar or cafe. The trolley ride is a hoot.Nightlife on the Riverwalk.
I went to San Antonio with my family right after Christmas two years ago and we had a blast! The Riverwalk is sooo pretty! If you can get a ride on one of the tour boats you should, they point out all the interesting places to go. There is a huge mall at the end of the Riverwalk with lots of good stores. Hope you have fun! Good luck :)