Natural swimming pool filled entirely by nearby natural springs. The water is (REALLY) cold, but it's worth a dive. Located within Zilker Park. Bring $8 to enter if you're an out-of-towner ($3 if you're a local).
Kayaks and stand-up paddle boards on Lady Bird Lake in Downtown Austin. No reservations needed, but be sure to bring your driver's license along with you to rent a boat. Tours are give Friday morning (book below).