Zilker Park (aka Zilker Metropolitan Park) is a city park that offers biking, trail running, birding (though I was the only birder in that park), Frisbee golf, walking, canoeing, kayaking, and exercise training along the Colorado River in downtown Austin. Canoe and kayak rentals are available. The park also has a Botanical Gardens area and hosts live music events.
The Ann & Roy Butler Hiking & Biking Trail goes along the bank of the Colorado River in a wooded area of the park. It is generally flat and well-maintained and paved in some areas. The trail crosses the river on several bridges and extend beyond park boundaries. Animals to see along the trail are Carolina Chickadees, Buffleheads, Lesser Scaups, Blue Jays, Redwing Blackbirds, Doves, Pigeons, Texas River Cooters, Mockingbirds, Coots, and an occasional Wood Duck.
WARNING! Hiking and outdoor related sports can be dangerous. Be responsible and prepare for the trip. Study the area you are entering and plan accordingly. Dress for the current and unexpected weather changes. Take plenty of water. Never go alone. Make an itinerary with your plan(s), route(s), destination(s) and expected return time. Give your itinerary to trusted family and/or friends.