To be quite honest I had learned that Lake Thun is not as beautiful as Lake Brienz. Which in my opinion is somewhat true, but not entirely. If Lake Brienz is 10/10 then Lake Thun is 9.9/10.
They are both different and stunningly beautiful.
Do take a cruise on Lake Thun as well, if you have the time, there is a beautiful Castle overlooking the Lake. It is called Oberhofen Castle and there is a restaurant at the Pier right next to the castle where you can sit by the lake enjoying some wine or having something to eat.
How we reached there:
Reached Interlaken West Railway station by Train. Walk out of the station following the sign boards, walk across the street to catch the Bus to Oberhofen. I cannot recall the bus no. Get down at 'Oberhofen a. T., Dorf' Bus stop and the castle is about 200m (2 mins) walk from there.
A couple of suggestions:
1. If you plan to cover Interlaken, Lake Brienz and Lake Thun all in one day then try to go for Swiss day pass, it also covers the cruise and the train and bus travels. Or if you have any other pass just check if it includes the cruise as well. If you have any doubts about it, just check at the ticket counter.
2. We did not have the time to take a cruise on Lake Thun, but you see if you can squeeze that into your itinerary. If not, then you could do what we did, choose the place on Lake Thun where you would like to spend time, we chose Oberhofen. Then from Interlaken West Bahnhof, take a bus to that place (if you are not going to be driving).
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.