Swiss pass/swiss card/half fare card

Welcome Jeannie! There are several options indeed. Looking at the travel plans you described, a 4 day Regional-Pass Bernese Oberland would be best, being slightly cheaper than a Swiss Half Fare Card. If you want to make (even more :-)) calculations, these tips may be of help. Keep in mind though that your plans may change, and in most cases you'll only know afterwards whether or not you picked the cheapest solution.
If you choose the Regional-Pass, you should use free travel days for the first 4 days, and buy a regular ticket for Interlaken-Basel on the last day. On your way from Zurich to Interlaken, you'll need a regular ticket up to Bern or Lucerne (depending on which route you pick), a discounted ticket Bern-Thun or Lucerne-Brünig, as that's where the discounted area of the Regional-Pass starts, and the remaining leg will be for free. Please refer to the Regional-Pass page for a detailed network map, specifics, purchase options and prices.

If you like traveling in 1st class and want to limit buying additional (discounted) tickets for your trips, you can look into the special 4+1 offer on the Swiss Saver Pass. This would cover all 5 travel days.

I hope this helps you decide!

A note to Marie: if you're looking for rail pass help for your specific situation, you can best create a new topic for this issue or continue your questions in your previous topic.

Sorry Jeannie, I missed your travel dates. It is indeed available from May. In that case I would pick the Swiss Half Fare Card. An alternative is the Swiss Card, which is only a bit more expensive but offers more convenience with the transfers to and from Interlaken West being included (no tickets required). It may even lower in price as I expect promotions to become available in the coming months. Just keep an eye on our website.