>> Interlaken - Grindelwald - Jungfraujoch - Gimmelwald - Lauterbrunnen - Interlaken
As Lucas suggested, including Gimmelwald in this day might be a bit much, as it is on the other side of the Lauterbrunnen Valley from the rest of your trip.
The route would be Interlaken - Grindelwald - Kleine Scheidegg - Jungfraujoch - Kleine Scheidegg - Wengen - Lauterbrunnen (all by train). At Lauterbrunnen you would catch a bus to Stechelberg, then a cable car to Gimmelwald.
To get back to Interlaken, you could return to Lauterbrunnen from Gimmelwald the way you came, or go up to Mürren in the cable car from Gimmelwald, walk through Mürren to the railway station (about 20 minutes), get the train to Grütschalp and the cable car down to Lauterbrunnen.
You would want to use the Swiss Timetable to check how long all this would take before deciding to do it.
If you want to stop off somewhere on the way back to Interlaken, instead of Gimmelwald I would suggest Wengen (which you will go through anyway). Wengen is a picturesque traditional alpine village with stunning views into the Lauterbrunnen Valley (see attached photo, for example).
>> St.Moritz - Tirano - St.Moritz
If you do this trip on regional trains, reservations are not required, but if you take the Bernina Express, reservations are compulsory, and can be made here:
Tirano does not have a lot to offer. You could consider taking the official Bernina Express train from St Moritz to Tirano at 9:30, which would get you to Tirano at 12:00, then leaving Tirano at 13:00 on a regional train. You might be interested in stopping off at Alp Grüm, or even going up to the Diavolezza Glacier on the way back to St Moritz.
The Diavolezza cable car is not included in the Swiss Travel Pass, but as you are staying 2 nights in St Moritz in September, you might be eligible for the "Mountain Railways Included" card if your hotel participates in this arrangement. With that card, the Diavolezza cable car is free. There is more information, and a list of participating hotels, here:
www.engadin.stmoritz.c h/sommer/en/bergbahnen -inclusive/
>> St.Moritz - Zurich
There is an alternative scenic route to Zürich, which would avoid backtracking along the sector between Chur and St Moritz (and is a bit quicker). To get this route in the timetable, you can put "via Klosters Platz" in your timetable search. Connections departing St Moritz at 47 minutes past the even-numbered hours from 8:47 to 16:47 might be most convenient, as they require only one change of trains (at Landquart).
If you would like to see some photos taken along this route, please see my post of Feb 28, 2017 - 4:36 on the following page.