Scenic Train / Scenic Stay suggestions?

Here is your best plan, IMHO.
23rd, go to Chur. From Luzern, you could get portions of the William Tell Express and Glacier Express routes. Take a lake boat to Fluelen, then a train to Goeschenen, change to a train the short way further up the mountain to Andermatt. Change to a train to Chur which is part of the Glacier Express route. Spend the night there. You can ride the actual Glacier Express, but each passenger has to pay a 33 CHF reservation fee.
24th, take the Berina Express to Tirano, Italy. This train also has a mandatory reservation fee with panorama cars. Then have some lunch and cross the street to the Nord station and get a train to Milan. It goes along the east bank of Lake Como. Spend the night in Milan.
25th, take a train to Venice, they run about every hour. That takes 2 1/2 hours, so you can pick the train that gets you there when you want.

Here's the SBB description of their panorama trains where "express" does not mean fast:
http://www.sbb.ch/en/leisure-holidays/holidays--short-breaks-in-switzerland/scenic-trains.html

We did the Bernina Express as a one day excursion from winter in Davos-Klosters to outdoor cafe in Tirano, Italy for a pasta lunch. It was a rest day well spent. I would have been in trouble during the film camera days where my film budget was a single 36-exp roll per day.

Sam/Edgar, Thank you for your ideas. I am exploring both options. I always like to hear what someone else. There is so much benefit from the experience of others.

You might wish to get off the beaten track by heading to Appenzell in eastern Switzerland. Trains go to St. Gallen and from there smaller ones into Appenzell. The town of Appenzell is surrounded by towering cliffs with many opportunities to hike and/or take cable cars up and hike about or back down. Check this out for information or Google whatever: http://www.myswitzerland.com/en-us/appenzell2.html Spend a day or two here hiking and seeing the countryside by the trains. Stay 3 nights and you receive a train pass which is quite a money saver. Strongly recommended: go up the Ebenalp on the cable car; take a small detour and cable car up the Santis mountain; and the Hoher Kasten as well. All have restaurants on the top to take a break. The views from up top look like a Grandma Moses scene. Have fun!

Thank you for the additional information. Clearly we will need to keep returning to Switzerland. There is so much to see!

I just wrote a long post with an alternative plan to Sam's, using the Bernina Express route but staying in a mountain village like Samedan and then in Varenna on Lake Como (instead of Milan). We did this to travel from Zurich to Venice in 2015, and loved it. The whole post was lost when I hit "send" as the website decided to kick me off---apparently I was logged in too long. I don't have time ( or the heart) to re-do it now, but let me know if you are interested and I will re-create it tomorrow.