Hello Andy T,
Using Bern as a base is an excellent idea! Not only is it my favorite Swiss city, but it also offers numerous opportunities to explore the mountains. There are up to eight trains per hour to Thun, with travel times as short as 20 minutes. Thun is the gateway to the Berner Oberland, the stunning mountainous region of the canton of Bern.
An additional 10 minutes on the train brings you to Spiez, from where you can head toward Kandersteg or explore the neighboring valley that leads past the Stockhorn mountain to Zweisimmen and on to Gstaad or Lenk. All of these are beautiful destinations in the Berner Oberland, offering amazing hiking opportunities.
One of the key advantages of using Bern as your base is its central location, which allows you to easily pivot in any direction depending on your mood, energy level, and the weather. If you have a list of mountain areas you want to visit, you can check the weather forecast the day before. Often, one area might have poor weather while another, even close by, is clear, allowing you to enjoy a great hike with beautiful views.
Remember, all of these train journeys are very scenic—something many tourists are happy to do for hours at a time. So, spending a few hours on a train each day in this setting is not a chore.
Here are some travel times from Bern:
Bern to Lauterbrunnen: 1h 22min
Bern to Montreux: 1h 37min
Bern to the top of the Stockhorn mountain: 1h 31min
To compare, popular mountains to visit from Luzern also take about an hour to an hour and a half to reach. So, traveling from Bern to Lauterbrunnen or other beautiful mountain areas is well within the same timeframe.
When people visit Luzern, they don’t plan to stay within the city limits for three or four days—they know they can easily do beautiful excursions. The same is true for Bern and many other Swiss cities.
The Swiss cities will be quite warm in September.
All we can go on is averages from past years, and a quick Google search will tell you that:
The average temperature in Bern, in September typically ranges between
10°C (50°F) in the early morning or late evening and 20°C (68°F)
during the daytime.
I am not sure what ‘warm’ is to the person who makes this claim, but these are pretty much perfect temperatures to me, allowing for wonderful hiking/sightseeing weather.
This is a great website for the canton of Bern, with lots of ideas for the region. You can find similar sites for each of your chosen destinations.
https://madeinbern.com/en/made-in-bern