Choucossi60
On day 2, the Swiss Travel Pass includes free travel only till/from Grindelwald or Wengen. You’ll need an additional (expensive) ticket from Grindelwald or Wengen to Jungfraujoch and back.
The STP grants 25% discount on this Jungfraujoch ticket, and this specific 25% discount is only available either on-site (e.g. at Grindelwald Terminal) or on the Junfrau Railways Website www.jungfrau.ch
Using a Swiss Half Fare Card for 120.- would be slightly cheaper overall, because this one grants 50% on the Jungfraujoch ticket, and also 50% on the journey Heerbrugg - Lucerne - Grindelwald/Wengen and back again.
So the decision STP vs. SHFC also depends on how much free museum admissions (which are only included in the STP) you would use, especially in Lucerne.