Where there are mountains there are generally lakes! Some are bigger than others and some are natural whereas others are man made dammed lakes used for hydro-electric power.
Lakes are the beaches of the mountains and there is something wonderful about swimming in fresh water and sunbathing on grassy banks. It beats salty dry skin and sand getting in everywhere in my book!
On smaller lakes you can splash about and play numerous games with kayaks, usually available to hire from the lakes' beaches. Large lakes offer a real exploration experience with often several tens of kms of coastline to explore.