I love Hawaii...probably one of my favorite places to vacation. Last December was my first time to the Big Island, a real treat...and a reminder there is a lot to see on this chain of islands.
Big Island
Only go here if you plan to spend a good amount of time. The island is larger than all others combined with such an insane range of climate zones (from snow capped peaks to lush tropical forests to stark lava flows to green and black sand beaches) that it would be hard to appreciate if you aren't willing to give it the time. Expect to do a lot of driving. I was only on the Big Island for
seven days and, trying to avoid the wettest weather, did not get to some of the sites that originally drew me there in the first place (most notably the volcano park, snorkeling off the Kona Coast, and ascending the Mauna Kea summit). Be sure to visit a Kona Coffee Farm....definitely worth a trip to sample the good stuff!
Maui
My favorite of the islands to frequent...doesn't hurt that my wife works for The Ritz-Carlton, Kapalua. Wide variety of activities and sites to see with not nearly as much driving as the Big Island. I also find it has the best eats of the island chain. Snorkeling off Molokini is definitely a highlight. I'm hoping to run the Maui Marathon this year...not sure if I'm going to try training by running up and down from Haleakala, but the thought seriously crossed my mind when I looked down from the summit last year (unfortunately I missed sunrise). I'm looking forward to checking out the Kapalua Adventure Center on my next visit (more hiking, zip lines, etc).
Oahu
I must have visited here during the perfect time of year because, when I stayed in Waikiki, it wasn't nearly as crowded as I had been told it would be. I consider it a great transition for vacations...the ease and convenience of a city, but the casual atmosphere of the islands. Sure it is the most touristy of the chain, but that can be fun (especially when you're in town because of a friend's wedding).
Kauai
Perfect spot for a honeymoon (first hand experience)...feels a lot less touristy than the bigger islands and it is by far the most beautiful (some really spectacular vistas from the coastline and the canyon). And you'll definitely want to see this one from a helicopter (only way to see the waterfall from Jurassic Park). The north side is very wet, so you'll probably want to stay to the south and drive to see the north.