Choose the Right Trail
If you're planning to mountain bike, choose a trail that has plenty of shade and that also has a swimming area nearby. Luckily, here in Austin we have lots of trails and swimming holes to choose from. If you need to stay in town, hit the trails at Emma Long or Barton Creek and then pedal over to Barton Springs for a dip. Rumor has it that Barton Springs is offering FREE admission today. And they have the best dressing rooms if you want to change into a suit. If you're willing to drive a little further out, my top pick is Mule Shoe Bend. The trail is super fun and has great shade. It also has a swimming recreation area right on Lake Travis. There's restrooms in the parking lot near the trailhead so you can change before swimming.
I sure do value sleeping in on weekends but with temps in the 100s, riding early is the safest and most comfortable time to ride. It's actually quite pleasant from 6AM-9AM. If you want to get a few more minutes of shut eye, pack up everything and load up the car the night before.
Use your Common Sense and Hydrate, Hydrate, Hydrate
Drink water before, during and after you ride so that you hydrate properly to avoid dehydration. I always ride with a Camelbak and have a cooler of ice water/recovery drinks waiting for me in the car. I pack the Camelback with ice so that by the time I hit the trail, the ice has melted and it's quite refreshing.