Sauna

A wood-fired sauna set close enough to the lake that you can walk from one to the other without thinking about it.

Heat first, cold second, rest third, and then repeat if you want to. That cycle is the whole method and it has been the whole method for a very long time. We have not improved on it.

The rest phase is the part most people skip and the part that does the work. There are chairs facing the water for exactly this reason, and sitting in one doing nothing for fifteen minutes is a legitimate use of your afternoon.