Interlaken, possible to do all of this in a day?

The answer to your question really depends on how much hiking you want to do. I was there last summer and spent two days, one day for each side. With your tight schedule, I would recommend taking the train/bus to Trummelbach falls, then take the lift up to Gimmewald for a short hike. Depending on your timing, you could hike to Murren. We did not go to Harder Kulm or do a lake cruise so not sure how much time that would take. My advice is to allow enough time to soak up all the beauty of Lauterbrunnen valley area where Trummelbach Falls is. The views are amazing, especially on a clear day. Have fun!!

If you get an early start you have time for Jungfrau and the Falls, and maybe Harder Klum. But the lake cruises take a big chunk of time out of your day.

But I would do Jungfrau take the bus to the falls from Lauterbrunnen and then hike back to Lauterbrunnen before returning to Interlaken.

That is way too much. If you get good weather simply go for a hike and enjoy the awe inspiring views. Take a picnic with you or stop in some hamlet on the way. You can combine it with the Trummelbach Falls but not much more.
If the weather is lousy then consider a cruise on a steamer.

david,

IMHO, you won't have time to do all that in one day. The trip to the Jungfrau alone will take at least half a day, as you'll need to include travel time from Interlaken to Lauterbrunnen, then to Kleine Scheidegg and finally to the Jungfraujoch. It will cost you a small fortune to get there, so I'd suggest spending some time there to get your "money's worth". It will take time to tour the Ice Caves, visit the Sphinx Observatory and perhaps venture outside if the weather is favourable.

You should be able to visit Trummelbach Falls on the same day. When you return to Lauterbrunnen at the conclusion of your visit to the Jungfraujoch, take the Post Bus to Trummelbach Falls. If you get back to Interlaken earlier in the afternoon, you should also be able to visit Harder Kulm, however that would depend on the time. The last trip from Interlaken is at 18:55 on workdays, returning at 18:40. On other days the last trip up is at 17:55, returning at 18:10. Another option would be to skip Trummelbach Falls and head straight to Harder Kulm after the Jungfrau.

I don't think a cruise on the lakes will be possible, unless you skip something else such as the Jungfrau.

Some compromises will be necessary (again IMO).

Essentially, you have one full day. You will not have time for all of these. I'd go to the Jungrfrau first thing in the morning. Then, I'd go to the falls.

You have a great list but only one day for this gorgeous area. You can maybe wait until you are there and see how the weather looks at the Jungfrau and then decide. But my vote would be to do Jungfrau joch first thing and then visit the falls on your way back to Interlaken. I would warn you that there can be long lines to get back down from the Jungfrau. I hadn't thought about that and was really surprised when I had to wait for a couple of trains before I could travel back down. If the mountains are socked in then you could switch things up and do a boat cruise, take the trip to Harder Kulm and visit the falls.

One of my all time favorite lunch spots is at Kleine Scheidegg. You will need to go there to take the train to the Jungfrau joch. There is a brat and rosti outdoor café that is amazing. Great views of the Eiger, Monch and Jungfrau and you can't beat the rosti.

Thanks everyone for the comments and suggestions. I may try to rework my schedule and spend two days in Interlaken instead of one.

You could also drive across the entire country of Switzerland in one day...but that wouldn't be much fun, would it? There are lots of things that are possible, but just because it's possible doesn't mean you really want to do it.