Ruby, Please, please, consider staying in the heart of the Berner Oberland (Murren or Wengen) instead of Interlaken, which is too far away. You will be spending an hour just getting to the perched villages in the alps. MUCH better to stay in the heart of it. Interlaken is a big town without much charm.

Personally, I would skip Zermatt.

Also, I should clarify something I said (about perhaps needing a car to get to the castle above Stein am Rhein). You don't need a car for this trip (in fact Wengen and Murren are both car free villages). The trains will take you efficiently to all these destinations. If you don't want to do the hike up to the castle at SAR, it is skippable -- really worthwhile only for the views of the village below. I would instead maybe visit the falls (check connections) or walk to the island monastery.

I just learnt that "the heart of the Bernese Oberland" is 40 miles away from the Western end (Les Diaberets), 25 miles from the Eastern end (Titlis), 15 miles from the Northern end (Brienzer Rothorn) and less than 2 miles from the Southern end of Bernese Oberland (boundary with Canton of Valais).
Interesting.

I didn't mean "heart of" in a literal sense. I was trying to impress upon the OP the importance of staying in the perched villages close to the Eiger / Jungfrau peaks, rather than Interlaken.

Ruby, Please, please, consider staying in the heart of the Berner Oberland (Murren or Wengen) instead of Interlaken, which is too far away.>

Yes the Berner Oberland is a much bigger area than the Jungfrau Region in and around Interlaken, Grindelwald, Lauterbrunnen, Wengen and Murren which along with tiny Gimmelwald (don't miss it - totally unique place) are about the only proper towns.

But to tourists the Berner Oberland has kind of taken on the meaning of the Jungfrau Region - but to Swiss like neckervd it means something far different.

Thanks for clarifying PQ.