There are stretches of the San Marcos river, within the city limits, that have amazingly good fishing with easy access. Rio Vista park is one example, with a half mile of fishable water. Go early this time of year to avoid the tubers. Launch and take out just east of the RR tracks. The park at the Lions Club Tube Rental is also available, but the water current is pretty swift and you may find yourself downriver.
One area I have recently fished is the canal at Stokes Park. A kayak/canoe, float tube, etc., can be launched at the canal and paddled upstream into some pretty nice water. There is very little current for a quarter mile, with many fish just waiting for a fly. The tree canopy is tight, so bring your shortrod. Some beautiful bass and sunfish are eager to put a smile on your face, right here in the river city.
Let's go fishing.