Well you have come to the right place. I have just updated our website section on Switzerland (with still more to come):
http://www.slowtrav.com/switzerland/index_switz.htmInterlaken is a lovely town. I would suggest you skip the Jungfrau. We spent two weeks in Grindelwald in 1988 and another 10 days in 1997 and are heading there for a week this September. We love this area. We took the train to the Jungfrau in 1988 and it was not that interesting. It is a long, slow ride to the top of the mountain. Then all you get is a glacier - and views.
This is what I would do:
1 day around Interlaken. I think there are boat rides you can take and there are probably some good mountain rides from there. Go to the Tourist Office website for Interlaken and have them send you tourist information.
http://www.interlaken.ch/The tourist information for Switzerland towns is usually very well done - lots of good info.
1 day to Grindelwald, Wengen and Murren. A narrow gauge train runs from Interlaken and in a circle thru these three towns. If you have a Europe train pass, it will not work on this train because it is privately owned. But you might be able to get a pass for all the rides in the area - or just pay for it.
You could take the train to Grindelwald, then the gondola either up one side of the mountain and down to Wengen, then train to Lauterbrunen, then gondola to Murren, then gondola back down and train back to Interlaken. (You could skip Murren.) Or, if you like to hike, take the gondola only half way up the hill then walk a few hours to Kleine Scheidegg, train from there to Wengen.
Wengen is a small town with no cars. So is Murren. Grindelwald has stunning views of the Eiger and is a lovely town.
Let me know if you want to do a day of hiking because I can get more info for you. I will also find out about the passes you can buy for rides in the region, because I need to figure that out for us (or if you already know, could you post the info here?).
Here is Pauline's big piece of secret information for Switzerland: Don't buy cow bells in tourist shops, go to a hardware store. They come in many sizes from goat to sheep to huge cow. You can also buy the leather collar that goes around their necks. They are cheaper and better in the hardware store.