hcsna would be my first recommendation to you. it is a beautiful and large park with nice big hills and lots of area to roam. they have a great trail system out there and the times i have been, i had it basically to myself. another place you might look into is
Pedernales Falls State Park because it has some good scenery and a good trail system. the developed sites can fill up and people frequent the falls for sight seeing, but the trails out to wolf mountain are typically empty, and the primitive sites are rarely taken either - at least everytime i have been there (at least 5 or 6 times).
or another place i have recently feel in love with is
Lake Georgetown's Good Water Trail. they have some very accomodating primitive sites with picnic tables and fire rings. hcsna has this luxury as well. most primitive sites will not allow camp fires - so keep that in mind!