If you are in Round Rock, it is worth paying the $60-odd for the Texas State Parks pass. As far as public land, there ain't a whole lot around because we like everything big, including our back yards. I think you can get into Huntsville SP without paying, but I am not certain. Check out the TPWD website...
Excellent advice.
I would certainly put the $60 on a park pass as well. Since hunting is down, park money is down as well. They have a lot of volunteer forces at some of the parks, but they ALWAYS need money. In fact, in the face of the current economic problems, the last park I went to over the summer the Park Rangers told me they were talking about shuttering some of the parks since they can't afford to maintain or upgrade them.
Since you are in RR, you have some great places (state parks) that you can get to easily. You can't miss Perdinales State Park, and it is an easy drive from Austin. Lots of trails, lots of water (pretty rare this far south!) and the park is quite clean with lots of places to roam and explore off the trails. There are
a lot of natural springs there in one area of the park, and just a few feet off the trail and you feel like you are alone.
Like any affordable place to to, skip these places during holidays. They can get pretty busy if you prefer total solitude.
Bastrop State Park is an easy drive for you as well. Great hiking trails, big pine trees, and some areas look positively "foresty". Another strange thing this far south.
I don't mind a bit paying the money to keep our nice State parks in shape. Here in San Antonio, we have a few "natural areas" they have just opened up, with more on the way. They have hiking trails, just north of the city.
You might want to check out Austin as well. This is a handy site, and it mixes city with State parks.
http://www.trails.com/activity.aspx?area=11538
Note that it shows the Bastrop trails ( 2 or 3 of them ) as well as the Perdinales trails. None of them are long, but if you make the Perdinales trip you will spend a lot of exploring time.
My last trip there this past summer required me to explore a large cut shelf in the limestone mini cliffs, and that seemed to take about 4 hours. We weren't far in, and actually didn't get far. But we had a great time!
Robert