Define "secluded". If you really want to get away from folks, I recommend
Hill Country State Natural Area in Bandera.
Closer to San Antonio, and with some options for quiet if you pick a good site, I'd recommend
Guadalupe River State Park. It's best on weekdays, particularly in the summer. Weekends are VERY busy/crowded/loud.
The lakes south of San Antonio are nice, but I wouldn't include "secluded" on my top 10 list of adjectives for them!
