Since you are staying in Bern, you may wish to go as far south as Montreux.
There are two obvious routes -
1.- via Gruyeres and Montbovon
2.- Via Lausanne (the main line)
There are some particularly nice scenic parts of each of these routes. Good weather, especially sunshine on the vineyards makes them special, but they are still nice on a cloudy day.
On route 1.- From Montbovon down to Montreux. The descent through the vineyards above Montreux looking over the lake is a wonderful sight. The train does switchbacks, so you can sit on either side.
On route 2.- The view from Grandvaux over the lake. This requires a bit of explanation.
As you near Lake Geneva, south of Palezieux, and just before you reach the lake, you pass Puidoux, then Puidoux-Gare on the left. Then the train turns to the right for Lausanne and passes through a short tunnel.
As you emerge from the tunnel, you overlook the vineyards, the lake and the Alps. You must sit on the left after Puidoux.The change from countryside to lake view is sudden, an has high impact...best on a sunny day.
The Swiss in the French-speaking region have a joke that people coming from the German -speaking parts of Switzerland are so amazed by the impact and beauty of this scene that they throw their residence cards out the window and move to Canton Vaud or Geneva.
The train proceeds above and through the vineyards to Lausanne. From Lausanne, there is a local train to Palezieux that stops at all the small towns.
When I was working in Geneva, for a day trip I would often go to Lausanne, then catch that local train as far as Grandvaux. I would get off there and take a trail from slightly above and to the west of the station, walking down through the vineyards to Villette, and then to Cully. Then, I would take the lower level (lakeside) train back to Lausanne. Good photos on a clear day. See attached from just above Villette(mine) and another from Panoramio in the Internet and a third from Wiki.
Do a Google search for Grandvaux Switzerland.. Some nice pictures.
Also, I have mentioned before, that there is a "wine train" through the vineyards from Vevey to Puidoux that has great views, especially on a sunny afternoon.
www.montreux-vevey.com/en/CL-ar/Culture_Lavaux_en/a ctivites_lavaux/trainv ignes