Personally, I would do what wanderlust suggested and stay on Oahu, there is so much to see and do on all the islands and the travel day really takes a day of vacation away from you. This is my little synopsis I send to friends and family who ask about the differences between the islands
Oahu
-Where the city of Honolulu is
-Where Waikiki Beach is
-VERY BUSY
-There are sections of Oahu that are gorgeous but what most people do is spend 3 or 4 nights here and then move on to a second more relaxing island for another week or so.
-Arizona Memorial is here
-Diamond Head crater is here(non active volcano)
-Hanauma Bay snorkeling area is incredible
-TONS of shopping, every thing from Tiffanys to
Walmart
-Some great beaches like Kailua
-North Shore which is awesome and laid back
-Dole Plantation/Zoo/other touristy kids activities
Maui
-gorgeous beaches
-high end resorts and condos (but lots in every price range)
-incredible dining options
-lots of celebrities live here or own second homes here
-can get busy but nothing like Honolulu
-the "road to Hana" is here, famous twisty turny ocean drive with many beautiful scenic places to stop, for exmple 7 sacred pools
-Iao needle and Haleakala crater (dormant volcano) are here, incredible place to watch the sunrise
-lots of good hiking
-Maui Ocean Center is here for the kids
-Historic Old Town of Lahaina, and old fishing/whaling port is here
-Some good upscale shopping in a few different places
Kauai
-laid back
-incredible secluded beaches
-lots of eco-tourism
-lush and very tropical
-some very high end places to stay
-ocean kayaking on the Napili coast line is a big draw for this island
-very peaceful but not quite as much to do as Maui
-Waimea Canyon also draws people as it is touted as the Grand Canyon of the Pacific
-Most movies filmed in Hawaii are filmed on this island and you can do a movie site tour to see all the diff locations
-Fern Grotto is here
Big Island (formal name is "Hawaii")
-very slow and laid back
-one of the most incredible beaches we have ever been to is here, Hapuna beach
-Lots of black and green sand beaches to explore
-active volcano is here but is on the other side of the island from where tourists stay
-You can visit the summit of Mauna Kea and there is an incredible star gazing here
-Waipio valley & Akaka falls are natural highlights
-You can do night dives with rays in Kona
-up country is cowboy territory (they are called paniolos) and it is always a kick for me to see the equestrian and farm side of the islands