I would plan all downtown outdoor activities for morning hours only. The river barge tour is definitely worth the money, but is pretty miserable after lunchtime. Even after sunset, the Riverwalk and downtown temps will be in the 90s. For historical interest, I recommend Mission San Jose and its welcome center (though I'd skip the film in the welcome center; it's pretty dopey), and take the Riverwalk to Main Plaza and step inside San Fernando Cathedral. Some of the original building has been uncovered in the chancel, and inside the left door as you face the altar is the sepulchre containing the remains of the Alamo heroes. The Menger Hotel is worth a walk-through as well. The hotel and bar hosted many notables. Teddy Roosevelt made it his HQ as he recruited and trained the Rough Riders, and there are ax marks where Carrie Nation took a few whacks at it (of course, the Menger bar is still here, but Carrie Nation isn't ;))