Switzerland for only one day - Where can I get to?

Go with Paris to Interlaken, then at Interlaken change for the train to Lauterbrunnen. At Lauterbrunnen change for the train to Wengen. Stay in Wengen. My first trip to Switzerland was in 2017 for two nights just to see. And I went all the way up to the Schilthorn early in the morning the day after I arrived and then flew from Geneva to Nice. I have been back 4 times since 2017.

Thanks Jazz+Travels! This is a great suggestion and I really appreciate it! But the problem is, after spending the day there, how do I get to Naples the next day? Do you mean I could fly out of Geneva? That might work... Train to geneva and fly to naples from there...?

Forget about flights from Bern. That airport has not had regular service in years. What you are probably seeing is private flights.
But that does not matter. Getting around Switzerland by public transit is trivially easy, so you can use any airport to leave from.

For Paris to anywhere in Switzerland look on www.sbb.ch. You will see times and prices there. Note that to get to Napels you could also go by train all the way. you will find those trains on www.sbb.ch as well. When comparing travel time take in to account the fact that for train travel it is fine to be at the station 10 to 15 minutes before departure. Hence Wengen - Napels would take you 7-8 hours with a flight from Zurich, and around 9 to 10 hours all the way by train

September is the month with the best weather for visiting Switzerland, but it is also the month with the least snow cover. But the peaks are always snow covered. If you splurge for a trip to the Jungfraujoch or the Titlis you will get to see a lot of snow up close...

Bern airport is getting scheduled services back this summer on Helvetic Airways, but nowhere useful to you - Spanish and Greek destinations.
Also charter routes to several southern Italy destinations on other airlines, but charter flights won't be of much use to you. They are probably tied in with package holidays, hotel inclusive- hence the high cost.

I get what you want to do and why. But let me be the voice of reason and say don’t bother. As Rick Steves says, don’t try to cram it all in. Plan as though you can always come back. Switzerland deserves its own trip, and you deserve the best of Switzerland. Focus on the amazing trip you already have.

I really appreciate all this input! David, I also appreciate your comments about maybe just skipping it. Hard to do since I have waited 50 years to get to Europe and I'm unsure how long it will be before I get to go again - but I hope that I will! I also don't want to overbook myself and spend more time on a train than in a location enjoying it. I will definitely consider that - whether i would prefer to have more time in Paris (I was cutting it down to just a day and half to add Switzerland) and then go relax on the beach in Amalfi. Hard choices...

Would I enjoy spending 8-9 hours on a train through the alps and down through Italy? Is it comfortable and would that be a nice way to spend a day, resting on the train and watching the countryside go by? I'm genuinely asking - it seems like that might be nice.

In your case, forget what Rick Steves said and follow your dream and get a brief taste of Switzerland; it will probably lead you to more visits like it did for me. You can fit more when traveling solo; at this pace you just need to stay hydrated.

This is a great suggestion and I really appreciate it! But the problem is, after spending the day there, how do I get to Naples the next day? Do you mean I could fly out of Geneva?

Yes, that is no problem; you can get 3 different direct flights to Naples from either Basel, Zurich, or Geneva, on Easy Jet.
With Easy Jet you just have to read and follow their rules for baggage allowance.
https://www.easyjet.com/en

I would not take the train all the way to Naples (its more complicated for a novice traveler with making connections to other trains and if one is late it could cause you to miss another) from Switzerland. Only if you were stopping some nights in Milan or Florence on the way.

I don’t know about you, but I personally would not enjoy 9 hours on a train, no matter how great the scenery. What you could do is break the journey into 2 segments, one day through the Alps from Luzern to Milan, then the second day from Milan to Naples. For your overnight in between you co not have to stay in Milan; you could break your journey from Luzern at Lugano, on a lake by that name; it is a short ride from there to Milan the next day for the train to Naples (a 4.5 hour journey if you select the fastest train).

Or you could continue past Milan and overnight in a smaller town such as Parma.

For the trip south from Luzern, you have the option of the Gotthard Panorama Express, which is a boat+train combination on the rout over the Gotthard Pass route. You can see the video and photos here:

https://www.sbb.ch/en/leisure-holidays/trains-boats/panorama-trains/gotthard-panorama-express.html

In September you may or may not see snow on the peaks—-there should be some in the distance, but I cannot promise. We did this route at the end of September last year and what I remember most of the scenery from the train is all the waterfalls. But there must have been snow melting somewhere to produce all that water—-probably the high glaciers.

We did hike through meadows with glorious views of the lake and the mountains across the way on Rigi, the mountain near Luzern. We actually stayed the night in Weggis, on the lake near Luzern (40 minutes away by boat).We rode a cablecar up Rigi directly from the town for our walk. The next day we caught the boat down the lake to Flüelen to catch the train to Milan.

https://www.rigi.ch/en

Maybe something like that would appeal to you.

With such limited time, you cannot have everything. I don’t know what your expectations are for snowy peaks, but it won’t look like wintertime even if you go as far and as high as Wengen. I just looked at the photos of our week in Mürren last September, before we headed to Italy, and while there was snow on the rocky peaks, it was small glaciers, not snow-capped mountaintops. Lots of rugged rock, with glaciers lying here and there.

Maybe you could take another day from your time allotted to the Amalfi Coast? We have never been there, but it doesn’t sound like the relaxing beach time you are picturing.