Although the drive from Austin takes nearly two hours, a trip to Enchanted Rock won't leave you or your dog disappointed, even if yours, like mine, would rather be swimming. The view from the top is wonderful, the climb is easy (but tiring), and there is a lot to explore.
Make sure to take plenty of water and try to visit on a weekday. On the weekend you'll be stuck in along line of traffic before you even get into the park, and once you get in you'll then have a challenge of finding a parking spot. But don't let that deter you. I do prefer a nice solitary walk in the woods, but the atmosphere at Enchanted Rock is very relaxing even with all the people. Take a picnic lunch and enjoy the day exploring the top and walking the trails.
When you leave it would be a shame not to head down to Fredericksburg for a walk through town and maybe a beer (or wine, if you prefer) before the trip home. By the time you're ready to head home, I'm sure your pooch will be ready to nap the entire way back. And if you drink too much beer, so will you! Probably better to let your dog drive home.