place i'll go: 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.
Gorges de l'Areuse, Switzerland The Val de Travers is a Jura mountain valley perpendicular to the main valleys (hence the name). The river Areuse has cut itself steeply through the soft limestone and clay sediments. It's one of those Roman bridges too.