We just were four days on Kauai and the 7 days on Big Island and it rained all the time. On Kauai, if waves are calm enough, snorkeling is always a great activity for a rainy day cause the fish don't care. We also did a bit more shopping and driving than usual, just a bit less sitting at the beach and relaxing. The waterfalls are more spectacular for sure during rainy times, though hikes can be a bit more difficult if ground is slippery.