Grindelwald to Lauterbrunnen is easy. You can take a train to Zweiluetschinen, then change there for the train to Lauterbrunnen. The trip takes about 35 minutes, but is pretty dull. A more interesting way would be to catch a train to Kleine Scheidegg (takes about 30 minutes), change trains there and travel to Lauterbrunnen (takes about 50 minutes). This second train travels through Wengen, so you could jump off there and have a look around. All up, the slower route takes about 90 minutes, but it's a lot more interesting because you are always within sight of the Eiger, Moench and Jungfrau. I can't remember which pass is best for these trips, I'll have to leave that for the better informed.
There are a couple of Falls in the Lauterbrunnen Valley. The Staubach is quite close to town and only take a few minutes walk to reach. That's the waterfall you see in all the photos of Lauterbrunnen. The Trummellbach are the famous falls that have the tunnels following the water course. Quite spectacular! The tunnels are very noisy, cold and very wet, so make sure you take your wet weather gear. It's a dead flat walk, but if you're in a hurry, take the bus. It should say "Trummellbach" or "Richtung Stechelberg" (Direction Stechelberg)
From Lauterbrunnen you can take the cablecar to Gruetschalp, then a short train trip (narrow gauge, I think) to Muerren.
So, to recap, go from Grindelwald to Kleine Scheidegg, change for Wengen, hop off there and have a look around, take the train to Lauterbrunnen, then the bus to Trummellbach, return to Lauterbrunnen, take the cablecar to Gruetschalpand and the Narrow Gauge to Muerren before heading back to Lauterbrunnen. This time, instead of returning to Grindelwald via Wengen, take the faster train via Zweilutschinen.
If I can make a suggestion - consider leaving out Muerren. It is a very pretty village and well worth a visit, but if you try and pack too much into your day, you may not enjoy it so much. It's up to you.
I'll post a "Fly Through" animation of the excursion later.