San Antonio's Downtown and Riverwalk (safe for night strolls) area is compact and walkable, including the Alamo. IIRC, there's a purple Via bus which will take you to and from El Mercado (free). There are some moderately priced "lesser brand" hotels along the Riverwalk. You certainly don't need a tour or guide for the Alamo, small in scope and with good signage and illustrated explanations.