Lake Thun, 13 miles long and 2 miles wide, is sometimes called the Riviera of the Bernese Oberland due to its mild climate. It is extremely pretty with its mountain backdrop and castles and chalets dotted along the shore. Any view in any direction is picture-postcard perfect. The steamers leave frequently from Interlaken to cruise along the lake. They depart from behind the West train station. The round trip to Thun and back takes around 4 hours, but you can hop on and off at different points of interest.
We had half a day on the lake but would recommend setting aside a full day to enjoy and explore properly. The steamers run on time and do tend to be quite busy. Especially useful for us was the children's play area - which they all had - a few toys, colouring pencils, and pictures worked a treat to keep the little ones amused.
Thun is the main resort and is located on the northwestern corner of the lake. It has a picturesque castle and quaint medieval centre, but is also a large, busy town spread away from the mountains and does not have the spectacular views and character of some of the smaller lakeside villages. We found it to be less interesting than some of the other places along the lake and would not recommend it as a stopping point - there are nicer places to see.
Spiez is a gentle little resort village dominated by its waterside castle and stunning views over the lake to the high mountains. There are lovely waterside restaurants and cafes where you can relax with a glass of wine and soak up the scenery - a stop at Spiez is highly recommended.
Lake Brienz is to the east of Interlaken and is the smaller of the two lakes. It is 9 miles long and 2 miles wide. A round-trip on Lake Brienz takes around 3 hours. It is said to be the cleanest lake in Switzerland and is beautifully set in a bowl with forrested slopes and streams tumbling down into the water. Lake Brienz is much less touristy than Lake Thun. Steamers on Lake Brienz leave frequently from the steamer port behind the East station. We spent a peaceful couple of hours at Brienz on the northern edge of the lake. It has a very quiet feel and is most famous for its woodcarvers, whose work can be found in souvenir shops all over Switzerland.