Itinerary help

I think keep it to two hotels. Do Bern as a day trip.

We were underwhelmed w/Bern - it was a ONE & DONE for us. Lucerne is one of my favorites in all of Europe. I'd NOT move for 1.5 days. Less is sometimes more. I'd stay in Lucerne and day trip to Pilatus if interested. I'd skip Bern, for us, nothing stood out as anything we've not seen before.

From Wengen you can hike two hours to Kleine Scheidegg, from where the Jungfraujoch train departs. Descend into the Lauterbrunnen Valley to see the Staubbach snd Trummelbach waterfalls. Trummelbach is a series of 10 waterfalls that are underground. You can also take the gondola from Stechelberg at the end of the valley up to Murren and hike to the mountain village of Gimmelwald— an easy one-hour hike on a paved pathway.
More adventurous is the Grindelwald First Cliff walk. For this spectacular hike, , take a cable car from Grindelwald up to First glacier.
I agree with the others about Lucerne—-arguably Switzerland’s most beautiful city— for the trip’s finale.
Lucerne’s mountain trips to up to Mt.Pilatus on the world’s steepest train ride, and Mt. Rigi are spectacular on clear days.
And the train trip from Lauterbrunnen to Lucerne is one of the best in Switzerland.

Have a great trip!

If you will spend 3 days in Lucerne, use one day on a mountain trip to Mt. Pilatus or Rigi if the weather is good. Lucerne is a beautiful city, but IMO 3 full days is a stretch.

I think before you fully decide you should look for lodging. Options are going to be slim, so see if you can find somewhere that excites you to stay before committing. To me, if I'm going to stay in Luzern I'd want to be around the historic district or in one of the villages along the lake, as the outskirts aren't that exciting.

Also keep in mind that August 1st is Swiss National Day, which is a really fun cultural day. It's like July 4th with fireworks but add in cultural music, parades, food, etc. etc. Personally I love being up in the mountains for this, as they do bonfires all over the mountains and the cultural celebrations are really lovely. A consideration you could do, if your chalet is available additional days is to stay in Wengen until August 2nd and then just go to Luzern for the day before you leave + that evening and then take the quick 1 hour direct train to the airport from Luzern. There is so much in the Jungfrau region to occupy you easily for a week - hikes, mountain viewpoints, car free villages, biking, castles, lakes, day trips to a city, etc. etc. If you can make it work to be in Wengen for August 1st I think you would really enjoy it and I can easily give you a one week itinerary that wouldn't leave you bored.

Thank you for all of the helpful replies! Lucerne looks very beautiful but lodging is quite expensive! Is it worth it or should I explore another city for our final 3 days? Having seen lakes and mountains while in Wengen, wondering what should be next?

The Lakes and mountains in Switzerland all have their own personality, and I never tire of the beauty- each has a separate personality. Experience as many as you can. Also, one never knows what the weather will bring, so give yourself as many opportunities as possible.
Have a great trip!

Try the Capsule Hotel or the ibis Styles Luzern in
the Altstadt— rooms are about $200 per night at the beginning of August.

FWIW, I LOVED Bern, especially in the evenings, when daytrippers were gone!
Such a beautiful, walkable city, to explore ancient alleyways and the two-tiered stores! The fountains are lovely.
There are many pubs and cafes.
And HALF the tourists of Luzern!
Walk to Rosengarten above town for great views of the Aare River bend!
Have Roesti (if you haven't already).