From Montreux, additional options would be an excursion by train to Geneva, and the option to visit to CERN:
https://visit.cern/
We have been trying to incorporate that into our Swiss trips but have not yet managed it. for some time they made it rather difficult to visit, but now it is not. There is a tram from Geneva.
The sports fans might like the Olympics Museum in nearby Lausanne:
https://olympics.com/museum
And if you visit Gruyères you might like to include the Chocolate Train:
https://www.myswitzerland.com/en-us/experiences/chocolate-train/
Montreux is 3 hours or less by train from the Zurich airport, with a single change along the way, at Zurich main station or Lausanne. The trains originate right in the airport terminal, so it is easy. If you choose to stay in Lausanne instead, there are direct trains with no change necessary. These towns—-Montreux, Lausanne, Geneva, and more—-are all right on Lac Leman. Some like to take a boat ride across the lake to Evian in France. I know you said you were not interested in lake cruises, but this is a short trip across the lake from Lausanne. Here is a rail map of Switzerland that shows the boat connections as well:
https://www.sbb.ch/content/dam/internet/sbb/en/freizeit-ferien/inspiration/internationale-gaeste/Geltungsbereich-Swiss-Travel-Pass.pdf.sbbdownload.pdf