This is Alta Lake in British Columbia, Canada. Alta Lake is located just outside of Whistler, and you can get there by foot or bike on the Valley Trail that starts in the North Village. The lake itself is stunning, and whilst you can lounge on one of its beaches, I would recommend getting a canoe and going out on the lake. The water is calm and from your canoe you get the best views of Whistler and Blackcomb Mountains, and the peak to peak gondola that connects the two.
Alta Lake is surrounded by forest and has a few parks on its shore, where visitors camp, picnic, have barbecues, or access the beach from to go swimming or windsurfing.