Garner State Park. My family has been going there since it was under construction (friends with some of the CCA workers). There's maybe 2 years of my entire life I haven't spent at least a weekend there. It's got a lot of sentimental attachment, as many of my greatest childhood exploits occurred there. A big secret: go in the off season. Wonderful, and peaceful! You may have an entire section of the park to yourself.
Lost Maples State Natural Area. Usually a day trip from Garner, with excellent hiking, and absolutely beautiful during the late fall color change.
Inks Lake. Drought does not affect this lake, and there are some prime camping spots that you can beach your boat right next your camper. A bit small, but good hiking and water activities, and smallmouth bass.
Pedernales Falls State park. I missed out on the most fun here, because my parents' generation told me it used to be a great natural waterpark if you had the required brains to avoid drowning yourself. Still, it's got a nice stream, a beautiful natural grotto, and some excellent hiking and back country camping.
Lake Somerville. (Birch Creek St park, since Nails is usually closed!). Good fishing lake once you learn it, and decent swimming, too. Enough shade from the post oaks, and good for boating, etc.
Huntsville St Park. A change of pace with piney woods. Good lake for kayaking, fishing, and maybe a swim. Best in the fall and spring, due to the humidity. Excellent mountain biking trails around the lake.
Colorado Bend state park. Hard to get to, and isolated, but that's a big part of what makes it nice. Fishing and boating and a ton of hiking opportunity. Camping is fairly primitive.
Palo Duro Canyon state Park (or whatever it is). If you're way up there, the canyon is a great place to stay. We liked taking short likes in the morning and evening when it wasn't so hot, to see the geography/geology.
South Llano river state park. A miniature Garner State park, effectively. Good fishing and swimming, with some hiking. The toobing is more reliable than GSP, really.
Balmorhea state park. Small, in the middle of nowhere, but very unique. The CCA pool built around the spring is great for a swim, and the creek and the wetland areas are very interesting to observe. Also a good place to "interact" with tarantulas.
Guadalupe River State Park. Sort of a poor man's Garner State Park. Needs more river frontage, but still a good place to relax in the water (when there is some!), and some good riverside shade and plenty of back country hiking, and adjacent to some interesting state natural areas.
There are more but so far these are my top ones.
Also, must include both our Texas national parks out west. Both excellent hiking, photography, and vista viewing parks.