Judith, as you can see from all these comments, everyone has an opinion about what works best for them, and the way they enjoy vacationing. Having someone tell you how you should vacation is a little like picking out a greeting card for someone else to give out. It's a tad odd, and not quite the right fit.
Which is why you need to decide how you want to see Hawaii. If you're interested in laying on the beach, enjoying peace and quiet, and staying away from crowds, there are plenty of places to go and enjoy. Just about any of the upscale resorts have lots of that sort of thing. Depending on the island, even some of the lesser-named timeshares have nice beach or pool areas to enjoy.
But if you want to get out and see the sites and scenery that makes Hawaii a very special kind of place, you have to decide which of those sites are important to you. If you want to see an active volcano, you have to be on the Big Island of Hawaii. If you want to see Diamond Head or Pearl Harbor, you have to be on Oahu. If you want to see the old whaling town of Lahaina, or experience the Road to Hana or the sunrise on Haleakala, you have to be on Maui. If you want to see the rainbows and taro fields of Hanalei, see "South Pacific's" Bali Hai, or experience the amazing scenery of Waimea Canyon (the "Grand Canyon of the Pacific") you have to be on Kauai.
There is no right answer, and no single trip to Hawaii will give you the full experience. You could easily spend months in Hawaii, and never see it all. That's why some of us keep going back time and again. Not only because there is something new around every corner, but the time of day, the time of year, and your frame of mind can make even the same experience feel like something completely new.
If I can make one suggestion, it would be to not limit your frame of mind to "one and only trip to Hawaii." Figure out what you want to see, and what it will take to do that. Then take your trip and enjoy the dickens out of your time there. If it's not enough, you can always make plans to return to see what you didn't see this time. The point of it all is to enjoy yourself. Everything else is just extra stress. And more than anything, a trip to Hawaii should be about NOT being stressed.
Enjoy yourself!
Dave