WIPES US OFF THE MAP??!?!? Do you know something I don't?
There is a lot to do in San Antonio, more than you can possibly do in 2 days. There's a good reason why San Antonio is the # 1 tourist definition in Texas. I would suggest you stay in the Hilton on the Riverwalk. If you're flying, there's no need for a rentcar. You can take Taxi or public transportation the 2 miles to El Mercado.
Another thing you'll want to do, especially since you're interested in architecture, is to go to the King William District. Have breakfast, brunch or lunch at the Guenther House on the grounds of the old Pioneer Biscuit company, then cross the street to see the art studios at the Blue Star art center. You can do a walking tour or a trolley tour of all the old restored houses in the King William District, either before or after you go to the Guenther House.
You might also see what play or concert is showing at the Majestic Theatre while you're in town.
Check out the Southwest Art center on-line to see what is happening there while you're in town. The Buckhorn museum is near the Alamo, within easy walking distance of your hotel, and is a great place to visit. Lots of old west memorobilia and stuffed animals, etc.
Lunch at Shilos (german deli within walking distance) or Jail House cafe (2 mile ride). Both are old San Antonio favorites with good reason. Warning: Split a meal at Jail house, portions are HUGE! They sell cinamon rolls that are the size of a manhole cover. As you might expect, most or all of the restaurants on the Riverwalk are over-rated and over-priced, many of them are just national chains (like Planet Hollywood, Hard Rock, Dick's Last Resort, etc. An exception, if you don't mind spending the $, is Los Canarios in the Hotel de la Mansione.
Best to avoid mid June through September, it's very hot.