Hi Summer0761 -
Welcome to My Swiss Alps.
You have chosen a very nice time of year for your trip, and are wise to allow several days in the Jungfrau region in case cloudy weather on high makes the trip up to the Jungfrau an expensive way to see the inside of the clouds.
You have also created an itinerary that avoids rushing around from place to place, trying to "cover" everythng possible.
Keep close tabs on the weather up high on the Jungfrau while you are there. Ask at the tourist offices, the train stations or your hotel/inn. the weather can change frequently in the mountains, and is difficult to predict more than a day in advance.
Please check these links, especially the parts about Interlaken.
Interlaken is an excellent travel base, but lacks the character and scenery of the mountain villages, such as Mürren and Wengen. Grindelwald and Lauterbrunnen are also nice. Lauterbrunnen is on the valley floor, but still has some nice views. In my opinion, higher is better.
I like Wengen. So does Kim:
Looking at your travel plans, you don't really need the convenient rail links from Interlaken to the rest of Switzerland. Bern is close by, so the extra travel time from Wengen ( for example) or Grindelwald still allows plenty of time to get to Bern, see the city, and return in one easy day.
Your proposed trip to Zermatt is not a one day round trip as you have planned it, so the extra time needed to get out of the valley to Interlaken is only for a one way trip on that day.
Have you looked at the time table:
and a map:
for guidance on the trip back to Zürich from Zermatt and for times in the Jungfrau region?
The timetable defaults to the fastest route, unless you insert a "via" or connection in the available box along with start and destination. There are many options from Zermatt to Zürich, including the day long Glacier Express as far as Chur and thence to Zürich, the long route via Montreux and Bern, and the more usual routes through Spiez then either Bern or Luzern.
In your travels, you may wish to consider the especially scenic old route from Brig to Spiez for part of one of your trips. That train route has a special train, called "The Lötschberger" which shows up if you tell the time table to select a route via Kandersteg. It takes over an hour longer than the default route through the fast new "basis" tunnel that ooes between Visp and approximately Frutigen, near Spiez. The scenery inside the tunnel is rather uninteresting.
The timetable, unless given the "via" connection through Kandersteg, will send you through the new tunnel.
The attached photo was taken on September 22, 2009, on an unusully clear and sunny afternoon near Mänlichen. Note the clouds gathering near peak of the Eiger ( and out of sight to the right) the Jungfrau.