This trip is in late April, correct? I don’t think you need to worry about “tourist trap” feelings in Switzerland at that time, but I could be wrong.
But if you would like to visit one of those other countries, The Bernina Express, as Sam suggested, will get you to a nice part of Italy—-Varenna on Lake Como. From there you could move to Milan for a night before returning to Switzerland. The best route back would be either the Gotthard Panorama Express train plus boat combo between Lugano (just north of Milan) to Luzern, or the Treno Gottardo between Locarno and Luzern.
Either way, I suggest you head straight to Chur from Zurich (with an overnight at one or the other is this is the day you land in Switzerland). Ride the Bernina Express to Tirano and on south another 1.5 hours to Varenna for 2 nights. Then to Milan, and either Lugano or Locarno. You will end up in Luzern and can spend time there before moving on to Interlaken. Spend your last night before flying out of Zurich in Bern, a lovely little city (we prefer it to Bern but that is not a popular view here).
Another option, instead of the Bernina Express, would be to visit Colmar in Alsace, a 2-hour train ride from Bern (change trains at Basel SBB). Colmar is in France, but Alsace has changed hands between Germany and France, so there is a bit of German flavor to it in the architecture and food. You get a taste of both France and Germany in one spot. And it is a lovely town, especially in the area around Petite Venise.