Lady Bird Lake
One of Austin's most redeeming, natural features is a placid river named "Lady Bird Lake" that separates South Austin from the rest of North Austin. Get on the lake early in the morning for some kayaking, canoeing, paddleboarding, or even sculling. There's also a well-groomed trail that runs along side of it for you to jog/walk/ride. And afterwards, once you've taken it all in, there are dozens of great local restaurants within walking and biking distance.
Get there early before it gets crowded and/or hot.