Hello Again
Thank you for your very kind words.
Have a good map of Switzerland in front of you.
Monitor the Weather Forecasts
Keep checking the Swiss forecasts (https://www.myswissalps.com/planning/practical/weather/forecast/).
I have looked at the forecasts for next week. Only Monday looks to be a complete washout weather wise. Sunday looks bad north of the main Alpine chain, but OK in the southern cantons of Ticino and Valais. That kind of situation is not uncommon.
If on Sunday morning you see that the forecast is reasonably good for Ticino canton) (that is the Lugano/Locarno area).catch the 08 18 IC21 train to Lugano; gets there about 10 00. Later go on to Locarno.
Finally return northwards on the Treno Gottardo which leaves Locarno at 33 minutes past each hour. These trains take the old slow route via Airolo. The run takes just over three hours. The trains at 13 33 and 15 33 serve Lucerne. Those at 14 33 and 16 33 go to Zurich, so a change is needed at Arth-Goldau.
Jolly boating weather?
Definitely yes, unless the weather is very stormy.
On all three relevant lakes I would upgrade to first class. The upper decks of ships are always first class only. You do get better views of everything which is going on! Ask the booking office at the landing stage to sell you the upgrades. You can do this until just before departure!
The 10 12 boat from Lucerne does a round trip from Lucerne to Fluelen and back. Gets back to Lucerne at 15 47.
On Lake Brienz (https://www.myswissalps.com/activity/boat-lake-brienz/) round trips from Interlaken Ost depart at 10 10, 12 10, 13 10 and 15 10. The round trip takes nearly three hours.
On Lake Thun (https://www.myswissalps.com/activity/boat-lake-thun/) the boats are based at Thun. Boats leave Thun at 09 40 and 11 40; they return from Interlaken West at 12 10 and 14 10.
Lake boats usually have restaurants.
Mount Rigi in Snow?
Yes, I have done that – and in late December! And I was only the second person to the summit that morning. (There was one other set of footprints!)
More seriously, I would take short walks on Rigi in the snow, but always stay within sight of the buildings close to the train station.
Mt Rigi: https://www.myswissalps.com/activity/rigi/
Alternatives from Interlaken.
The obvious suggestion is to visit the ‘Federal City’ of Bern (https://www.myswissalps.com/town/bern/); almost all the historic sites are close to the station. The Bundeshaus,, built on a promontory above the river Aare, is spectacular. For the (real live) bears take the number 12 trolleybus heading for Paul Klee Zentrum and get off at Bärenpark.
On the way to Bern I suggest you take the number 21 bus along the north sure of Lake Thun to Thun town (https://www.myswissalps.com/town/thun/). From there take a local S4/S44 suburban train which takes a more rural route (via Belp) than the main line.
If the weather is better south of the main Alpine chain, go to Zermatt, changing at Spiez and Visp. At Zermatt, perhaps go up the Gornergrat for great views of the Matterhorn. (50% discount for Swiss Travel Pass holders.)
Zermatt: https://www.myswissalps.com/town/zermatt/
Alternatives from Montreux.
Zermatt or Saas Fee (https://www.myswissalps.com/town/saas-fee/)are also possible from Montreux. The only change is at Visp. The number 511 bus goes every 30 minutes Visp to Saas Fee
Lausanne (https://www.myswissalps.com/town/lausanne/) is a beautiful city built on hills above Lake Geneva. The Swiss Travel Pass is valid on the boats across to Ėvian in France, and these workaday ferries do have a reasonable service all year round. For these ferries take the Mėtro down to Ouchy. (STP is of valid on this unusual line.)
Upgrade on trains to first class?
I do not think this will be necessary. I do not think there will be a lot of people travelling next week. The Easter holidays are over and winter sports have largely (not completely) finished.
I take your point about luggage accommodation in the ‘Belle Époque’ carriages. They are nice to LOOK AT, but they are not 21st century comfort! In particular, no air conditioning!
https://www.myswissalps.com/travel/train/classes/
The Freedom to Change your Mind.
As I indicated in one of my replies to your previous thread, I sometimes change my mind as to where I am going while I am on board a train, bus or boat. This is sometimes triggered by a change in the weather, especially if it is from bad to good.
I can recall TWO occasions when heading northwards through one of the long Alpine tunnels the train has come out into unexpectedly fine weather! On both occasions I immediately decided on a trip up a mountain.
Best wishes for your trip.
Roger