San Miguel has a very high elevation - around 6,000 ft above sea level. In August, you should expect the weather to be in the 80s during the day and in the 50s at night. I would suggest bringing a light wrap for the night.
No, we do not recommend renting a car. San Miguel is a small city that is easily walkable. Most destinations are only a 5-10 minute walk or a quick taxi/uber ride.
For the wedding activities I would suggest wearing heels with a block or low shoe. The city has cobblestone roads I would suggest wearing comfortable shoes when possible.
Your passport, walking shoes, a light jacket for the evenings.