I don't think this has been mentioned yet. "The Saga" is a cool short film projected on the San Fernando Cathedral each night, at 9, 9:30, and 10PM. Free admission. Just stroll up and watch. There are a few chairs, but most people stand or sit on the ground. It's really beautiful and one of the more unique things to see downtown. There is usually a little snack stand set up if you want a bite.
Best of all, it's dead simple to find. When you exit the convention center, take a left down Market Street and go about .7 miles. That's it. You can't miss it. Should be a nice stroll this time of year. Just note that our daily high temps of 70's and 80's can dip as low as the 40's in the evening, so dress accordingly.http://www.mainplaza.org/san-antoniothe-saga/
(NOTE: The movie plays on Tues/Fri/Sat/Sun, so don't bother going Wed/Thurs!)
Edit: Oh, and I apologize in advance for the number of tourists who insist on taking flash photos....of a light show. SMH