The Thunersee is a wonderful and large lake near the Alps. If you want to enjoy good views you should take a trip to Spiez, a community at its shore. It has a nice castle (Schloss Spiez), and you can have some food next to the water. In December and January, it is recommended to arrive early as the mountain Niesen throws a large triangle-shaped shadow on the city after 2 pm.
Spiez is accessible by train. From the train station you’ll have to walk down the hill, but don’t worry you don’t have to do the walk up again. Spiez is one of the places at the Thunersee that has a jetty. Ten ships are circulating on the lake and are part of public transport. For a fair price you can enjoy a wonderful boat cruise back to Thun.