Taking a train from interlaken/lauterbrunnen in switzerland to Venice in the first week of October. Anything interesting during the way that we can explore for a few hours or even spend a night at before we hit Venice?
I would say Stresa, on Lake Maggiore, is worth an overnight for a quick trip to the Borromean islands on the lake.
Or, à bit closer to Venice, maybe Bergamo or Verona but I've never been so can't advise on detail.
Lake Garda too.
I second balso, if Stresa is on the route. If Bellinzona is on the route, I'd do that.