A week in Switzerland after Paris?

Lucerne and a town in the Berner Oberland like Grindelwald, Wrngen, Murren. Easy to connect by train and gondolas/lifts.
A must see if weather is clear- the Jungfrau.

Forget Geneva and go directly to Lauterbrunnen and plan trips from there; for example go to the top of the Schilthorn from Lauterbrunnen. You should consult some kind of guide book to make sense of the geography and to reference the sites/cities/attractions in Switzerland mentioned here. Guide book will help you decide where you want to go based on what piques your interests; the same if you want to tour France instead.

With seven nights I would spend 2 in Luzern and 5 in the Lauterbrunnen Valley. Specifically, I would stay in Lauterbrunnen because it is central with access to both sides of the valley. You can spend your days riding lifts and trains and taking nice easy (or hard!) hikes. Rick Steves' Switzerland guide covers the area very well, but don't stay in his recommended Gimmelwald. It just is not convenient.

If you have 6 nights, make it 2 Luzern and 4 Lauterbrunnen.

We always get an apartment in Lauterbrunnen, but many here recommend the Hotel Staubbach or Hotel Oberland. The whole town is convenient to get around.

These are very helpful. If I travel by train from Paris to Lausanne for two days, then train to Lauterbrunnen for four days, should I go back to Geneva to fly home, or go to Zurich or Basil? Is one more scenic en route?

You said Lausanne - did you mean Lucerne? A bit of a difference, although both are nice.

You are more likely to find non-stop flights back to the US from Zurich. There is a rail station at the Zurich airport which makes it easy.

I agree with Laurel. I absolutely loved Lucerne (divided by a river and on a lake and surrounded by mountains) It might be best to end in Lucerne because if you use the Zurich airport it is about an hour away. Lauterbrunnen is stunningly beautiful.

Lausanne is near Geneva and on Lake Geneva. It's known for wine and cheese among other things. It's nice, but I believe the real stars of Switzerland are Lucerne and Berner Oberland(Lauterbrunnen) Watch the weather forecasts. In both Lucerne and Berner Oberland there are excursions you can take to the top of mountains, but they only seem worthwhile if there is visibility. In Lucerne, I really enjoyed the boat ride across the lake to Vitznau and the train ride to the top of Rigi.

Baconc, I am doing the opposite in May/June next year. I fly into Zurich, train/gondola/train up to Murren for 4 nights. Train back to Lucerne/Luzern for a night. Train from Lucerne to Zurich HB (Main train station) and catch a TGV directly to Paris Gare de Lyon.

Check fares out of all the Swiss cities back to your home airport. For me, Zurich is by far the cheapest, but it's possible that it will be different for your routing.

The forum has persuaded me. Lucerne will be my base for 3-4 nights, then fly home out of Zurich. I'd welcome recommendations for hotels in Lucerne that are near train station and moderately priced.

I don't know what you consider close to the train station, but try Drei Konige. https://www.drei-koenige.ch/de It is well priced, and we liked it very much. The staff was helpful, and breakfast was excellent. I believe it was 1/2 to 3/4 mile from the train station and about 1/3 mile off the river. It had local charm and was in a more residential area.

Baconc, I chose Hotel des Alpes for my stay based off of suggestions from this site: https://www.desalpes-luzern.ch/en/welcome.
Located just across the river, 10 min. walk from train station.

I was also looking at Hotel Krone Lucerne, https://krone-luzern.ch/en/.