Here's an idea you might like to consider. This route combines sectors from both the Wilhelm Tell Express and the Glacier Express and can comfortably be done as a day trip from Zürich.
The routing you need for this trip is:Zürich HB - Arth-Goldau - Göschenen - Andermatt - Disentis/Mustér - Chur
To get this route in the SBB timetable, put Zürich HB as the "from" station, Chur as the "to" station and Arth-Goldau and Andermatt as "via" stations. This trip will take 5-5.5 hours depending on which services you select.
Between Zug and Arth-Goldau, the train travels along the Zugersee (Lake Zug), with wonderful views, often from an elevated position, of the lake and the mountains on the other side, including the Rigi. Sit on the right.
The sector from Flüelen to Göschenen is part of the Wilhelm Tell route.
Between Göschenen and Andermatt, keep a lookout on the left for a glimpse of the Teufelsbrücke (Devil's Bridge - see attached photos).
From Andermatt to Chur is part of the Glacier Express route, travelling across the Oberalppass, which is the highest point of the route at about 2,033 metres. This sector can be travelled on the official Glacier Express train, or on local trains - so pay attention to which service you choose.
If you choose an official Glacier Express service, a reservation is compulsory. This will be shown in the timetable information on the SBB website by a little icon of the letter "R" within a square, and the type of train will be described as GEX. It might be a good idea to mention here in the forum what sort of a ticket or pass you intend to use, as in some cases a small supplement is payable between Göschenen and Disentis-Mustér.
Both sides of the train on the Glacier Express route have great views, and if the train is not full, you can move from side to side during the trip. (At least, you can do this on the local trains).
You could have a meal and a wander around Chur for an hour or two before returning to Zürich by the most direct route (passing through Landquart and Sargans), which will only take about 1.5 hours. The tourist information office in Chur railway station can give you a map of Chur with details of the local sights. I have also attached a few photos of Chur.
By the way, you can also do the Golden Pass route as a day trip from Zürich. To return from Montreux to Zürich, you take the most direct route via Lausanne, which will only take 2.5 hours compared with the Golden Pass route which takes about 6 hours from Zürich. A bonus with the Montreux to Lausanne sector is that it takes you along the lovely Lac Léman (Lake Geneva) and through the beautiful Lavaux terrace vineyards.