Aug 7, 2012 - 11:05 AM
Travel time from Interlaken to Jungfrau is 2 hrs approx, including transfers. I would suggest, if the weather is good, spend time in Jungfrau and enjoy the views and activities.
While returning, you can return via Lauterbrunnen also. If you have some time, you can take a bus form Lauterbrunnen station to visit the Trummelbach falls. Or you can come back to Interlaken and enjoy the sights / shopping there ...
Also, Kleine Scheidegg, from where you change trains, is a quaint little place, and you can enjoy the vista there also by taking a later connection to Lauterbrunnen / Grindelwald on your way back.
Hope this helps.