You have ten nights not counting the night before your flight, so I think you might enjoy taking 2 from the Berner Oberland and spending them on Lac Leman as Laurel suggests. You can use the intervening day to visit the popular Chateau de Chillon, and/or hike in the vineyards above the lake.
https://www.lake-geneva-switzerland.com/lavaux/hiking-in-the-vineyards-of-lavaux/#
https://www.novo-monde.com/en/vineyards-lavaux-switzerland/
You can take the scenic Golden Pass route from Interlaken Ost to Montreux. You will be going in the “right” direction, coming down from the pass toward Montreux with the panoramic view of the lake below you.
https://www.myswitzerland.com/en-us/experiences/goldenpass-express/
Montreux, Vevey, and Lausanne all have their advocates. (We have not stayed in any of them so I have no recommendation to offer). I will note that Montreux is within walking distance of Chateau de Chillon (1.6 miles on the lakeside pedestrian path) if you are interested in that.
For the night before your flight, depending on the time of departure, you could move closer to the airport but still not stay in downtown Geneva (unless you want to). We stayed in Morges, 40 minutes from the airport by train, before our flight; chosen because of the Audrey Hepburn connection.
https://www.morges-tourisme.ch/en/V1243/in-the-footsteps-of-audrey-hepburn