We have been to Kauai, Maui and Big Island twice, all within the last 4 years. The addiction started as a last minute trip when we realized we had enough air miles and hotels points for a quick 5 night trip to Kauai right before Christmas, the next year was Maui and we stayed in Wailea, then we fell in love with the big island and went there two years in a row. Both DH and I had been to Oahu, he lived there in the 90s (with the Navy) and I had made a couple trips back in the 80s, so this March will be our first trip back there to take our DD (who is a Disney-a-holic and 15 now). We just dream about Hawaii all the time now and our special "Island Time".
There are things I love (and hate) about each island. For relaxation the Big Island is my fav, but I hate that it is so spread out, DH and I are waiting for DD to go off to college so we can go back to our favorite hotel the Mauna Kea on the big island, but DD feels it is a little "slow" for her. For family time I prefer Maui, it just feels like it is geared towards families, Kaanapali is a great area and I love that it is close to Lahaina (great for restaurants and shops), but if you do end up planning another trip and go back to Maui, check out the Grand Wailea (pricey though). I feel like Kauai is the prettiest (but it gets the most rain), the Napali Coast on Kauai is amazing. We have done so many things on each trip (snorkling, ziplines, stand up paddleboard, whale watching, seeing volcanos, hiking through lava tubes, swimming with dolphins, luaus, segway tours) and as our daughter gets older it is fun to see her pick out what she wants to do each trip, the first year it was all about going ziplining now she is looking forward to the SPA at Aulani! Sorry to ramble so much, but as you can see, this has become an addiction. Enjoy your planning!