Grab a sail boat and head to the nearby reservoir, because the water's always warm. It's easy to find a secluded spot to lay out in the sand (just go during the week), or take a relaxing float in the reservoir.
For a state that has not many natural bodies of water, Grand Lake boasts the state's largest. Take a pleasant drive through Estes Park and arrive refreshed in about 3hrs. Rent a boat at the marina, or doggy paddle right in from the sandy beaches.
Congress Park's close to where all the twentysomethings live in Cap Hill, there's less of a chance that when you go, it'll be overrun with families and their screaming children. The pool stays open 'til 7p.
Just under 3hrs away from Mile High, this all-natural mineral pool stays around 90-degrees year-round, which feels about room temp in the Summer. It's the length of a football field, complete with a diving board and water slides.