Seefeld Lake is a lovely place to get away from the hustle and bustle of daily life. See if any water activities are available here, or simply soak up the peaceful ambiance.
You're spoiled for sightseeing choice when staying in Sachseln. Seefeld Lake is a good place to start here, but there are plenty of other worthwhile sights to consider. Reichenbach Falls, Lake Lungern, and Melchsee-Frutt Ski Resort are close to the body of water and will top off your day nicely.