Aloha!
Something to consider is the length of time you'll be on each island. Due to limited time frames on each island, you might consider taking a shuttle bus to your selected activities. For O'ahu, I'd suggest making your car reservation but cancel it if you decide to bus it instead (you'll get to enjoy the views but won't have the same freedom).
I'll give you a list of shorter activities and longer activities by island. Approximate times are given for O'ahu, but they are my guesstimates. :
O'ahu:
Shorter: Parasailing at Kewalo Basin. It's located minutes from the cruise ship port.
Arizona Memorial at Pearl Harbor...best if you get there very early (long line) and refrain from carrying purses or bags (you may check them with security for a fee, but that takes a few extra minutes) Total time with transportation plus tour...3 to 4 hours depending on how much time you spend at the museum.
Pali Lookout...gorgeous views of the windward side of O'ahu. Total time including transportation...2.5 hours (Note: many longer tours, including the bus ride to the Polynesian Cultural Center, will include this stop)
Hiking Diamond Head...near cruise ship...moderate walk/climb, very well maintained, great views. Total time: 3-4 hours.
Surf Lessons, "aqua cycling" or just hanging out at Waikiki Beach. Waikiki Beach is very close to the cruise ship area.
Shopping (flea markets to extremely high end): O'ahu has the best and largest array of shopping venues in the islands.
Longer activities: Polynesian Cultural Center...gives you an idea of how vast Polynesia really is. You'll be introduced to the many different cultures of Polynesia, see dancers of all sorts, learn to start fire, watch guys scale tall coconut trees, etc. If you take the all day package, you'll be treated to a luau and a wonderful show that includes a fire dancer.
North Shore: The waves might not be too big till Fall, but you'll be introduced to the surf culture and laid back lifestyle.
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Can't give time estimates for the other islands because I don't know where the ship will dock and I'm not familar with travel time there
Kauai:
Medium time: Poipu Beach...absolutely beautiful
The fern grotto (pretty)
(Don't know time): Zip ride thru a rain forest. Heard it's a thrill!
Longer time: Hanalei and the north shore...Hanalei Bay is gorgeous. For upscale dining w/ a view of the bay, eat at the Princeville Hotel. A fun casual place to eat is at Zela's? Beach House. (Hanalei is far from the cruise ship, I think, and you might get hungry)
If you have a really long time on Kaua'i, see the Na Pali coast via kayak or helecopter (these excursions are expensive, but there are no roads to these areas...talk about unspoiled beauty!)
Waimea Canyon (the "Grand Canyon of the Pacific"): Very magnificent. It's on the opposite side of the island (from Hanalei). If you take this route, you'll get to visit Koke'e, a forested mountain area, and see beautiful Kalalau Valley. You'll also catch a glimpse of Ni'ihau, a private island. If you have all day, you can check out Polihale Beach close to sunset.
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Lanai (don't know well): Shipwreck Beach, Golf at Manele, a beautiful forested area at Koele
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Maui: Sunrise at Haleakala...add on the bike ride down Haleakala
Drive to Hana (all day experience)...make sure you actually stop at the lookouts...those are the best parts of the journey. On this drive, you'll get to visit the Seven Sacred Pools.
Drive the west coast of Maui...You'll see the islands of Molokini, Kaho'olawe, Lanai, and Molokai...really an Island feel.
Evening: "Ulalena" show in Lahaina. If you see one show in Hawaii, this is the one to see.
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Big Island (It would depend on where you dock)
West side (Kona, don't know much) Golf, Spas, Sunsets
East side (Hilo) Longer excursion...Volcanoes National Park
Shorter excursions:...Pacific Tsunami Museum...Rich in history, very sad stories of lives lost, and interesting scientific information
Akaka Falls
(I believe the Black Sand Beach is on the Southern tip of the island. It is beautiful, but I'm not sure if you'll have enough time)
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I know how you feel about choosing activities. We're visiting Alaska for the first time and are trying to select our activites. So much to do, so little time...
Hint: Don't try to do it all. Just pick a few things and enjoy them to the max. That's what we're going to do!
Have a beautiful time!