You'll be in the middle of the hill country.
It's a good time to fish and a great time to visit. You can fish the Guadalupe near gruene (green) for Guadalupe bass and grab a beer at the dancehall. The bass are not big, but fun and upstream closer to canyon dam there are trout that guys get excited about (they're generally bigger than the bass, which seem to run to about 14"). The blanco has smallmouth, and the llano and Colorado are all options with some bigger fish. You can also rent a kayak and float the San Marcos, which is a neat river and starts at the aquarena as a large spring (this place used to rent, but they'll refer you: http://tgcanoe.com/site/98de0983baa8451 ... s.php#2772
None of the above are great secrets, but are all good options. A lot will depend on what the rains have done-there's been a lot the last two weeks. As for access, if it's navigable, you've got up to the high water mark. Outside of the trout section of the Guad, you should easily find your own stretch. PM me if you want other specifics. Austin offers a lot, but it's now a giant hipster Mecca. McMurtry is usually at the Continental Club on Tuesdays, and that place still feels like the town I grew up knowing. Peaks and valleys.
Enjoy the area! I'm 3.5 hrs south, which is not the hill country.