Switzerland/France 16-night July -- Advice Needed

I am a big fan of longer stays and I think your Wengen/Luzern split is great because I love the Berner Oberland so much, but it is nice to at least see Luzern. You could go 5 and 3 nights each, I suppose. But if you love mountains and mountain walks and vista, spend as much time as you can in Wengen. :-)

Our first trip to Paris was 7 nights. We've had 4 more stays of 2, 3, 4, and 6 nights. You can definitely enjoy a full 7 days and 8 nights. Include a day trip or two to break it up. Versailles and Giverny for example.

Thanks so much, Laurel!

By the way, a train to Paris from Luzern is not out of the question. Takes about 5 hours with one change in Basel. It is probably faster than flying what with checking in, security, airport transfers, etc.

July I believe is lavender season, maybe 4 nights in Provence then a train ride back up to Paris for the remainder of your time.
Enjoy your trip.

Laurel beat me to it. The train from Luzern to Paris is only 4 1/2 hours if you make an additional change. And you go city center to city center. Much simpler than traveling to an airport ( you said "fly from France to Paris but presumably mean Zurich?) and then going through security, waiting to board, and at the other end needing transport from CDG into Paris.

If you need lodging recommendations for Wengen, we loved Hotel Bären. Very small, more like a restaurant with rooms upstairs. But the rooms have balconies and the food there is superb ( we took half board and it was a bargain).

Thank you all so much, I hadn't even considered the train across the countries. Lola, you are correct that I meant from Zurich to Paris -- I have corrected that typo in my original post! I really appreciate the lodging recommendations!

Six nights in Wengen is just fine from my perspective.
We typically aim for 10 days in the Berner Oberland region, realizing that rain, fog, overcast will affect perhaps a third of the days, leaving maybe 6-7 good hiking days. Here are our favorite hikes and some maps:

Mannlichen to Kleine Scheidegg (and even further to Wengernalp). Train back to Wengen. This is the BEST hike for a clear day.
Grutschalp to Murren, pretty much parallels the train tracks.
Top of Almendhubel, go right, generally heading above Winteregg, then clockwise back down to Murren
Top of Almendhubel, go left, towards Schiltalp, then Spielbodenalp, then to Murren
Murren to Sprutz Falls, under the falls, up the opposite side to Spielbodenalp, then back to Murren
Lauterbrunnen to Trummelbach Falls. Enjoy the falls and the walk through the valley
Schynige Platte to First (6 hrs, but great! Get to First before supper, when cable car closes.)
First to Bachalpsee and return (shorter version of #7)
First to Grosse Scheidegg and bus back to Grindelwald
Grindelwald, cable car to Pfingstegg (take the Rodelbahn luge ride there!), hike to Berghaus Baregg (If it rains, there is the Lauterbrunnen museum, Trummelbach Falls inside the mountain, eating, shopping the huge Coop in Interlaken Ost.)

Maps:
https://terrelledwards.files.wordpress.com/2012/01/hikingmap1-3-2.jpg

http://ontheworldmap.com/switzerland/ski/jungfrau/jungfrau-summer-map.jpg

http://www.pfingstegg.ch/index.php/en/timetable-prices-4/wanderkarte

I believe you will be better off and happier with the train. Discounted tickets for the international journey can be purchased 3-4 months th advance. I find the Swiss rail site SBB.ch easier to use than any of the French ones. But trainline.eu does a good job too.

Here is the website for our favorite place in Wengen:

https://www.baeren-wengen.ch/en/

Not fancy but comfortable and friendly. The reception is not staffed 24 hours so you need to be a bit independent. Frau Brunner is a welcoming hostess who greets everyone at their tables at dinner. We never met Herr Brunner as he is busy in the kitchen. One of the dishes we had there was a lime-ginger risotto which I have tried to duplicate to no avail. It is not typical "Swiss" food but fresh and modern, and they use local products as much as possible. Two kinds of butter at breakfast, one from a farm just down the hill.

I am almost talking myself into going back when we are in .Switzerland next summer, but we have already rented an apartment in Mürren. Maybe we will visit the Bären for dinner one evening.

Thanks Lola and Shoe!

Paris is always great, but with 8 nights to play with I personally would make one stop, either Alsace or Burgundy, for perhaps 3 nights.

A car would be helpful to explore either of those regions, but you can rent there.