We did the Dunns River Falls and Green Grotto Caves Excursion. The Dunns River falls tour was great...we did the climb with our 7yo daughter and 10yo son... they were both pretty nervous in the areas that have really fast flowing water but we completed the whole thing and were really proud ourselves. The falls themselves are beautiful, the guides were friendly, competent and fun, and we really enjoyed the climbing adventure. We bought the video for $40 and were pretty happy with it.
Definitely wear water shoes (although the rocks are well scraped and free of slippery algae) and be prepared to get 100% soaked on the climb. You'll need to have your hands free and won't want to have a backpack or anything on you that isn't waterproof. Be prepared for huge crowds and a strong sales push for photos, video and tips. There were also aggressive sales in the market by the falls, but they left us alone when we said that we didn't have any cash. If you are, however, looking for Jamaica souvenirs it looked like they had a great selection. We had a nice jerk chicken lunch there that was reasonably priced and they take American dollars everywhere.
There was a huge group on our tour of the Green Grotto Caves so it was tough to hear our guide at times, but the caves were really cool and the kids loved seeing the bats. When we could hear our guide he was really fun and had lots of interesting stories about the caves. I loved the grotto.... It is spectacular, and we enjoyed seeing the absolute dark in when all of the lights were turned off. (We had someone in our group who was claustrophobic so our guide accommodated him on the stairs with a flashlight... he was fine most of the time though). We took some fun photos and enjoyed the post-tour drink before heading back on our tour bus. There was a snack bar and a little gift shop there where you could pick up postcards or snacks if needed.