On our last trip we had friends who did the Molokina snorkeling adventure and raved about it. I've snorkeled several places in Hawaii and as Penny said, it is amazing there.
Kauai is a tough call: the beaches are beautiful, but it would be a shame to miss Waimea Canyon or the Na Pali coast. Personally, I think Kauai is the most scenic of the Islands--I would opt for a tour of the island . . .any tour.