Wake up early and head three hours north to Tenorio National Park. Inside the park is the famous Rio Celeste Waterfall with its turquoise water. Be careful; getting into the park can be tricky. Full directions to Tenorio National Park here.
Once you arrive, a 45-minute hike takes you through a well-maintained trail and down a built-in staircase to the Rio Celeste Waterfall. I promise the picture above is not photoshopped; it really is that blue. The water gets its color from a chemical reaction between the minerals of two rivers converging at the Teñideros point.