Let's get there early and spend the day to catch some morning,late afternoon and early evening light. You can spend as little or as much time there as you want. Bring a lunch for an afternoon picnic, snacks and water and wear good hiking shoes. You'll probably want a tripod also.
Here's a link if you're interested: http://www.tpwd.state.tx.us/state-parks/enchanted-rock
Looks like a nice hiking path, vernal pools, Moss Lake, Walnut Springs and Enchanted Rock Cave, not to mention the main attraction, Enchanted Rock....lot's to see and shoot. Maybe even a nice sunset :-)
Entry fee is $6.00. I believe that's per person.