you can also try this site www.101thingstodo.com
then put in the island you want. There are also many tour operators on each island that cater to cruise ship passengers as well as island tourists...try a search on hawaii activities, through yahoo search. there are even many booths that sell a huge variety of tours on each island.. they get a small kickback for the bookings but sometimes have better prices than booking direct. Lahaina, Maui has many booths selling every tour you could think of right on the main tourist drag.