Clear Lake in Oregon - absolutely gorgeous. Plan to take in the 5 mile hike around the lake. An underground spring at the north end of the lake is where the McKenzie river begins. The lake color is amazing! That is where Daniel proposed to me.
Travel: Austria's Green Lake in the Hochschwab Mountains is a hiking trail in the winter. The snow melts in early summer and creates a completely clear lake. The lake has a grassy bottom, complete with underwater trails, park benches, and bridges.