We actually enjoyed Hamilton Pool Preserve more outside of the preserve! There is an easy trail leading out to the stunning Pedernales River (the river we got engaged on!) which was more quite and serene. You do have to book day passes at about $12.
The original plan, or Plan A spot Harper had for our engagement was here! The plan was a sunrise hike with a hot cup of coffee to reward us at the top and then a picnicked breakfast. However, the morning we woke up to was 40 degrees with 30 mph winds. This led Harper to just tote my ring along with us while it dodged my hands reaching in his backpack for snacks. As Harper says, I was supposed to get my enchanted rock at Enchanted Rock! This spot has some of the most spectacular views of Hill Country we were able to see on our short trip. You will need a reservation for the hike which I would classify as moderate just for the quad workout of going straight up. This hike would be much more challenging for those scared of heights. (I'm looking at you Christy!)
This place comes recommended by our Air B&B hosts and is about 9 miles out. I can't speak first hand to the quality, but if you haven't had Texas BBQ, any is better than none!