The Rocky Mountains
You can bike to Vermilion Lakes in 15 minutes max from downtown Banff. Vermilion Lakes is made up of three separate lakes that are a few minutes apart in distance. The water is from the natural hot springs located on Sulphur Mountain.
You can pack a picnic and chill out on one of the three docks!
Lake Minnewanka is one of my favorite lakes in the Rockies. It's a short drive from Banff or Canmore, you can watch meteor showers here at night or the northern lights.