Deb, I agree with Tim about trains.
As for what to do in the area, yes, you can do the Jungfraujoch trip, but many many days the top of the mountain is in the clouds or might be once you get up there. I'd recommend instead to do the wonderful trails, all easy, but fantastic.
If you have 3 days in Murren, here are the 3 "classic" hikes in the area,
and they follow the 3 great ridges.
The first goes from Grutschalp to Murren. In your case, you want to hike Toward the mountains, so you should take the train to Grutschalp and hike back. From Murren, you can go down to Gimmelwald or you
can hike uphill on a diagonal from the Murren cablecar station, and eventually end up at the restaurant at
Spielbodalp. There you can get a drink and sit at a picnic table and watch the
cows. You can also get to Spielbodenalp via the Sprutz Falls route. About a third of the way up the trail branches near a hut. Take the upper one, you go to Spielbodenalp. The lower one goes fairly level into the woods and eventually to Sprutz Falls, which you cross under, climb up the trail on the opposite side, and end up in Spielbodenalp. You can see that trail and others here:
http://www.gimmelwald.com/pics/gimmelwald/hikemap.jpg
The second goes from Mannlichen to Kleine Scheidegg. The walk itself is
about 2 hours, and we save this for the sunniest day, because of the panoramic
views. (Leave in the morning--most afternoons the peaks cloud up!) Take the
train from Lauterbrunnen to Wengen, then a 5 min walk to the cable car
station, where you take the cable car up to Mannlichen. Once up, you can look
left at a 30 min up and back trail to a lookout point. If you look right
there is the main trail that leads toward the Jungfrau mountain, which takes
1.5 hrs and you end up in Kleine Scheidegg, where you can catch a train down
to Wengen and finally Lauterbrunnen. Great panoramic views all along the
route. This trail is so easy you could do it in a wheelchair.
It takes about 20 min train from Lauterbrunnen to Wengen, another 20 min cable car to Mannlichen, 1 and 1/2 hr hike to Kleine Scheidegg, 40 min train down to Lauterbrunnen.
The 3rd classic trail is from First to Bachalpsee. Take the Lauterbrunnen
train down to Zweilutschinen. Get off to switch to the train going up to
Grindelwald. Once in Grindelwald, walk about 20 min further into town to get
to the cablecar station, where you take it all the way up to First. From there
walk to Bachalpsee. There are trail signs, and it is about 45 min each
way. On a sunny day, you can get great pictures of the mountains reflected in
Bachalpsee, which is a small lake. Just Google image "Bachalpsee" and you'll
see what I mean.
If you want to turn this hike into a 6 hr hike (and this will be more strenuous than the rest), start at Wilderswil, take train up to Schynige Platte, and walk toward the Jungfrau mountains. There's a refreshment hut half way, then finally get to Bachalpsee, then 45 min more to First. Start early or you'll miss the 5:30 or so last cable car down from First to Grindelwald.
Finally, if you get a foggy or misty day, you can walk from Lauterbrunnen to Trummelbach Falls, which is about an hour each way (or take the PostBus). Google image it to see why Trummelbach Falls are worth seeing, and the nice thing is you can actually see them just fine on a dreary and foggy day.
Allen
