need help with last minute itinerary !!

I would stick with Wengen. Much more charming than Grindelwald. You could do the Jungfraujoch by taking the train to Kleine Scheidegg and then up the mountain for your visit (weather permitting then descending take the Eiger Express to Grindelwald Terminal and then hop the train for the short ride to the village center if you want to see Grindelwald. Then return to Wengen by taking the Männlichen Cableway up to that scenic area and on down to Wengen by the cableway on the other side. All of this is covered by the Berne Oberland Regional Pass referenced above, with up charges for a few segments. The Swiss Travel Pass does not cover the high mountain lifts; it only gives a discount the same as you’d get with the HFC. The BO Pass covers your journey from Luzern to Wengen.

If the weather does not look good for your Jungfraujoch trip, you might just enjoy joy-riding using the BO Pass for the route I mentioned sans the Jungfraujoch segment. The views are amazing even when the weather is not terrific.

It might be worthwhile to get a Swiss Half Fare Card which gives you a 50% discount on full fares such as from Wengen to Montreux/Lausanne and Zürich to Luzern. The HFC also gives you a discount on the BO Pass.

Be sure to set up an account at www.sbb.ch/en and link your HFC to it. Get the app on your phone and use it to find travel times and options.

Stay in Lausanne. Much prettier, IMO, than Montreux.

Thank you so much. How do you think about this instead? I am thinking of staying in Wengen my last 4 days because I heard it's beautiful. On my last day, I'm going to train to Wengen → Basel → Paris on the same day.
Something like this:
6:30–7:00 AM — Leave Wengen
↓
~10:00 AM — Arrive Basel SBB
↓
12:34 PM — TGV Lyria Basel → Paris
↓
~3:38 PM — Arrive Paris Gare de Lyon

Dani,

That’s a long day, but we’ve done something similar and will again this year. Just a note: the connection in Interlaken to the Basel train is from platform 2 to platform 7 or 8 and you have 5 minutes, so be ready to move! Or take an earlier train from Wengen, like the 6:43, and plan on the 7:54 to Basel from Interlaken Ost. Gives you 30 minutes, which is a lot but better than missing a connection you are counting on… That’s how we play it.

The swiss travel pass gives a discount on high mountain places of 25 to 50 percent depending on where you are going. The validity map gives the details.