Zermatt, day trip from Interlaken??

Interlaken to Zermatt is a two hour train journey, so it doable. Other good day trips to consider are Gruyere, also the Golden Pass train to Montreux, and on to Lausanne.

Brendon, One piece of advise if you decide to go to Zermatt for the day is to make sure the weather is clear there before you leave. Many days Zermatt is socked in with clouds and you can't see the Matterhorn. I have been to Zermatt several times and to me, the village itself is way too commercialized and over run with tourists. If you are in the Berner Oberland for only 3 1/2 days, you will have more than enough to do in that area. Assuming the weather is good, take the time to travel up to Grindelwald and catch one of the Gondolas (First or Mannleichen) up to the top of the mountains. There are good restaurants with wonderful views and some of the best hiking in the world (for all levels). I would also highly recommend going up to the Lauterbrunnen valley. That time of year, the waterfalls are roaring off the cliffs and the wildflowers are in full bloom. In that valley, you can also catch a gondola up to a revolving restaurant (the name excapes me) and there is also a roaring waterfall inside a mountain.

From Mürren, it is a 3-hour journey each way. And even if it is clear when you leave, the weather can change in an instant. The Matterhorn can be shrouded with clouds even on a clear day---it makes its own weather. So you might go all that way and not see it at all.

Thanks everyone for the info. We decided to stay in the BO and go to the Jungfraujoch, Piz Gloria and Trummebach falls.