A third for Camping Jungfrau (great set up with spotless facilities) and all the suggestions so far (definitely the Jungfraujoch - it is expensive but you'll only do it once); I'd also recommend getting the little zug up to Schynige Platte (you can take the train from Lauterbrunnen to Wilderswill to where you catch the little zug) where there is a lovely walk along the ridge (just don't look down) and a nice gardens depending on your thing. Great walking is a trail from Grindelwald to SP (quite long but stunning). Also walking from Lauterbrunnen from Wengen - easy uphill in and out of shade so pleasant in the heat. Another walk is up to Murren - again nice, easy to follow and you can get the train down.
A fun day out is the Shilthorn cable car up to the "James Bond" rotating restaurant - great views.
Don't miss the Tummelbach falls - incredibly powerful, thunderous waterfall through the rocks.
If you need to stock up with food before you get up to Lauterbrunnen there is a large CO-OP in Interlaken.
Lauterbrunnen is a great place to spot the base jumpers launching off the top of the cliffs...I wouldn't be brave enough
Horner Pub is a good place to hang out for some beers and I vaguely recall there is a decent cafe further along which sells good coffee and has wifi.
If the weather is rubbish and at all chilly then I'd recommend a fondue at the Hotel Oberland - yum!
Anyway sorry for rambling it's just we have stayed there several times in the past as we love it so much.
Have fun, CJ