Laura, this will take all day (so the earliest you can do it is tomorrow), and you need to reservve the bus (phone up, or at a rail station). I recomend taking a PostBus OVER one of the big Alpine passes. You can do a circular route, over one pass and back over another. the busses all stop for a 5-minute break at the top of the pass.
For example: Lauterbrunnen (train) Meiringen (bus) Grimselpass, Furkapass Andermatt. Then back by train via Luzern, which is a scenic route.
See here: https://www.postauto.ch/en/excursion-tips/grimsel-pass
For times, go to the SBB site: https://www.sbb.ch/en
And do a query Lauterbrunnen to Andermatt via Grimselpass and it will give you this route.
E.g. 08:02 depart, arrive 12:20, or 09:32 arrive 13.20.
Then lunch break, and look up Andermatt to Lauterbrunnen via Luzern
E.G. depart 14:29 arrive Lauterbrunnen 19:25.
Or, you can save some time and cut out the second pass: Lauterbrunnen (train) Meiringen (bus) Grimselpass (train) Brig Lauterbrunnen.