Hawaii was my all time favorite vacation we have taken. If we could do a week at a time, or could find a really good airfare we would be there more then Disney. Our trip was 17 nights plus one overnight flight. One week on the Big Island, 1 week on Maui, and 3 nights on Oahu. I loved the Revealed books (Maui Revealed, Big Island Revealed, etc) We were able to stay in 2 br ocean front condos with ocean views for the first 2 weeks, booked through vrbo.com and the oahu nights were at the Hawaii Prince at $95 per night priceline. (also water view, marina) We pricelined all our cars for the trip using biddingfortravel.com to decide on bids for the hotel and cars. We ate a few meals in the condos, but mostly ate out. On the Big Island we did a boat trip/snorkeling by Captain Cooks monument, that included a dolphin "show" as we left that was amazing. They were leaping and spinning all over the place. We snorkeled at Kahalu'u, went around the entire island, including Volcano National Park, hit the beach, explored Kona, did a Helicopter ride over the volcano and falls. In Maui we did the Iao Needle, a little museum near there, Lahaina with a lot of shopping and looking, The Old Lahaina Luau, the Hana Highway, Haleakala, a whale watch etc. In Oahu, we shoped, went to Pearl Harbor, and beached. (that beach was ruff) and went to Diamond Head. The entire trip was about $10,000 for the 4 of us. That includes about $3600 in airfare from Newark to Oahu, and three interisland flights, and about $500 for the helicopter. This was April of 2004, so i'm sure prices will vary. The weather in Hawaii changes more day to night then time of the year, so all times are good, just make sure you stay on the part of each island that has the weather you desire.
From the East Coast I wouldn't do the trip for less then 2weeks. Between the travel time and the airfare the longer the trip the better your bang for the buck.
Happy Planning, and if all works out we may be planning a trip out there Summer of 2010 for our 20th Anniversary.
Hope you can make the trip! Happy Planning.
Donna