We did our anniversary there last year
There are some really nice B&B's in the old King William District not far from down town. Or if you want a really nice hotel try the Emily Morgan Hotel. It is literally across the street from the Alamo and close walk to the Riverwalk. Our suite on the 7th floor looked down into the Alamo courtyard.
Go tour the Alamo...downtown.
For entertainment, if you like Dixieland Jazz, Try The Landing. It is Jim Cullum's place.
Take the old Spanish Mission tour.
Tour the King William district...neat old (for Texas) homes.
Witte Museum (pronounced witt-ee)
Eat at Shilo's Delicatessen downtown by the river..Rubens, split pea soup, homemade rootbeer.Etc...
La Villita is near downtown with lots of old houses converted into a wide variety of shopping opportunities in a close area without fighting auto traffic...art galleries too.
Great Chinese food in a neat historical location Hung Fong up Broadway near Witte Museum
Old Spanish governor's 'palace'.
Fiesta Texas and Sea World can be a lot of fun
Bullethole Gun Range
River City shooters at Blackhawk gun range for IDPA and IPSC
If you like basketball...S.A. Spurs
Go up Hemisfair Tower...even if you don't want to eat at the restaurant.
Institute of Texas Cultures
Natural Bridge Caverns 40 minutes north just west of New Braunfels.
Ride the narrow guage railroad at Brekinridge (sp) park.
San Antonio Zoo
Japanese Water Gardens
I have lived in Central and South Texas all my life and I still enjoy all these places. A Google search should get your contact info and about everything. The Chamber of Commerce and/or Tourist and Visitors could probably send you lots of info too. More Google. There are places where your can spend a LOT of money and places which are nearly free. It has a great mix. Good luck and have fun.
Avoid all the crummy places and situations that come with being in any big city. S.A. is the tenth largest city in the nation.
Finally, San Antonio has A LOT of very good Mexican Food Restaurants. It has been my experience that the plainer the building and the cheaper the prices the better the food. Drive around just south of downtown and take pot luck. Look for a lot of locals eating there. The places on the Riverwalk are overpriced and underseasoned...nowhere near the best of the lot. Try it for breakfast AND for lunch or dinner.
Have a great time. We do every time.
Overall you will find the folks in S.A. to be very hospitable and friendly...assuming your treat them similarly, which I don't doubt.
More people have been killed in Ted Kennedy's abandoned cars than have been killed in Guantanamo