Even at the very end of September, the temperatures were still hovering in the mid 90s the entire time we were there. The Riverwalk is fairly well shaded and while touring Mission San Jose, the guide made sure to steer us to shade while he was giving his talk but the walk down to the King Williamm Historic District was hot and so was touring the other missions. Try to plan activities such as the Alamo or visiting the missions for the earlier morning hours when it's slightly cooler, bring along a hat and some water.
In areas around San Antonio you may find fire ants. For this reason if you visit the San Antonio Missions National Historical Park, which includes missions in more remote areas it is best to stay on the walks. Also walk carefully on the walkways, ramps, and steps around the missions as some are uneven and can even be slippery. When the San Antonio River rises, flash floods are common and can be deadly. The mission trail south of the Mission San Jose is usually closed if the river is in danger of overflowing its banks.
If you go in the summer be prepared for some Texas heat.
Drink plenty of water, wear cotton clothing, and use sunscreen.
It was 102F when we were there and it was only May.
Even the steers know to stay in the shade.
Flash Flood Warnings! Water is one of the most dangerous forces on Earth and when it hits South Central Texas it is no joke! Look at the picture near my home!