Hello Youkohai42
There are two possible tickets:
- two one way tickets, one for each day
- two Saver Day Passes, one for each day
You can get the tickets from SBB: www.sbb.ch/en
I would not go to either Lauterbrunnen or Interlaken; the latter place is not in the mountains. Travel to Grindelwald or Mürren if you want to experience the mountains. Travel back from Interlaken to Luzern on the Lucerne Interlaken Express, stay in Lucerne for a few hours, then travel to Zurich Airport with a change at Zurich HB.
Kind regards
Roland