Barton Springs, Austin, Texas

In the midst of an Austin summer, Barton Springs is heaven on earth. Imagine the biggest swimming pool you've ever seen crossed with an old-timey swimming hole. Spring-fed, the water is cold, but when the sun gets relentless, it feels just perfect. The pool is in a gorge with a natural rock bottom (quite slippery with algae, so watch your step), and it's a few football fields long, so even when it's crowded, there's room to move. As you enter, you cross a steep lawn where you can park your towels before jumping in (just don't be surprised to see some sunbathers topless, as it's legal in Austin). There's concrete around most of the pool with stairs to get in and a shallow part (three feet) with a rock beach at one end, so even the smallest small fry can splash around. There are even lifeguards on duty. At $3 per adult and $1 for little kids, it's a bargain. There's even a small indoor museum with info about native marine life like the Barton Springs salamander and a miniature train next door for the young'uns!