It's been a couple years since my last trip, but I've been to Hawaii many many times (8+ trips?). Each island has something different to see and explore as you'd expect, and one week is not really enough to see and appreciate them all. It would be easy to spend a week on each island, but in my opinion I would say don't try to go to more than two islands, as it will take away from the relaxation aspect you're looking for and you'll have to deal with the airport, packing/unpacking, etc probably more than is comfortable for a week.
Oahu - Honolulu especially is very touristy, so be warned, but there are also excellent restaurants here and other non-touristy things all over this island that get frequently ignored. Hiking here is amazingly beautiful and easily accessible. Driving the coast to the North Shore is easily done and very beautiful as well. Sort of touristy but incredibly moving is a trip to the Arizona Memorial. Diving is good all over Hawaii, including Oahu, but some memorable places are the lava tubes and there's a WW2 minesweeper wreck which was also interesting. I've only dove Oahu once so I'm sure there is much more to see. Some also dive Hanauma Bay here, which is an insanely popular and often crowded snorkeling spot. Better for snorkeling rather than diving, but can be done as a shore dive.
Kona - Kona is MUCH less crowded than Oahu. Where Honolulu can look like just about any big city with it's fast food chains, chain steakhouses and seafood restaurants, Kona is quiet and relaxed. The Big Island also has the volcano, which is a good day trip. Underwater the topography is unique with lots of lava formations, black sand in some places, and tropical pacific marine life. The one thing that Kona has that is special is the manta rays. You do this as a night dive and the mantas come in to feed off the krill that are attracted to your lights. An amazing experience that I will never forget.
Maui - If I had to pick one island, this would be it. While it doesn't have the manta ray dive, the south side (Kihei / Wailea area) is incredibly relaxing, a great place to escape from it all, and the diving here is amazingly beautiful. Launching from Kihei,
Mike Severns Diving is probably one of the best operators I've ever dove with and I really enjoyed their attention to service and the environment. Molokini is a beautiful area to dive, the back side especially if conditions are good, but there are many good dive sites here, and a lot of shore diving is available for the adventurous. Top side, Roy's is a great restaurant, the road to Hana is a great excursion, Haleakala crater is also very enjoyable, and beaches and beautiful coastlines abound and are abundantly available for you to explore.
Kauai - Many people consider this their favorite island. It's very lush here, rains almost every day, and diving is really nice, too. I've only been to Kauai twice, dove once. Also very peaceful and relaxing. The day I went diving there were so many turtles I felt like you could walk on them like stepping stones. While you'll see turtles almost everywhere you dive in Hawaii, there seemed to be more here that day. None the less this also marks the one and only place I've ever seen frog fish mating. Very weird! Kauai also boasts its famous Napili coast, which is an amazing hike with very dramatic views. Camping is possible along this hike as well, as it can be very long if you want it to be.
Lanai - I've been here once, and dove Lanai from Maui. This place is very sleepy and very much an escape from the rest of the world. I don't know if there are dive operations that launch from Lanai, but if escapism is what you want, this is it.
It's possible I just don't know all the islands well enough, but my
favorite is still south side of Maui. It's grown a bit over time, but it's still a beautiful place to go and relax, and with a car, you can explore lots of places. Lahaina is the primary tourist area, and it may be fun to go see this area to check out these beautiful resorts, but the rest of the island offers so much natural beauty that you can fill your time being amazed every day at something new.
Hope this helps!
