Now you are down to hard choices and individual preferences.
I think I might go for 3 full days at the Jungfrau region, then go on to Luzern for a day and 1/2 or thereabouts.
Stay at Wengen, Grindelwald, Mürren or Lauterbrunnen, not Interlaken
From my perspective, a sunny day up on the easy trail from Männlichen to Kleine Scheidegg beats any other thing you will be able to do.
August is high season, so it might be hard to do this. But, in September or late June or maybe July, I would go to one of the above names villages on schedule. If I got great waether early on, then I'd try to move to Luzern sooner. If the weather was not so clear, I'd try Bern, Brienz, lake boats, Lauterbrunnen Valley floor and hang around waiting for a good forecast.
If you ever hope to go back, you ought to at least get a day in Luzern to appreciate how much you'll have to look forward to. ;-)
For the Wengen/Männlichen region, check the pictures in this thread, toward the end. There is also nice one of Pilatus from Luzern.
Also, this thread shows some of the southern part of the lake, with a few nice pictures:
Also, some pictures of/from Hammetschwand, near Luzern. Currently, due to construction, a bit awkward/slow to get there by public transportation. Maybe next trip.