Honolulu is where you go if you want to be in a big city in the tropics. It's just too busy a place for many, like me.
The other islands have far fewer people and, in my opinion, a nicer pace of life for travelers. Kauai, Maui and the Big Island all have major airports and are easily reached. My advice would be to try to do both islands if you have more than a week (say 8 or nine days), but with less than a week focus on one. Either of these two islands will be enjoyable, and you can always go back and explore the other one.
My favorite island is the Big Island, which is beautiful and has some really interesting sites, like Volcanoes National Park, coffee plantations, the world's tallest mountain (as measured from the ocean floor), and great scenery. And it's large enough that you actually can do some real exploring.
All of the Hawaiian Islands have a windward side (facing northeast), which are the wet, green lush parts of the islands. The leaward side (southwest facing) is dry because it is in the rainshadow of the volcanoes that formed these islands. So visit the windward side during the day, but stay somewhere on the leaward side so you have the dry warm weather.
The road to Hana is beautiful if you get to do it on a nice sunny day. It's a long, slow curvy drive, but the scenery is lovely. If you do take it, just take your time. It's not a race. The journey is what the Road to Hana is all about -- Hana itself is just a small town. My favorite place on Maui is Haleakala, which is well worth visiting.
My blog has posts on the Big Island and Maui, with lots of photos if you're interest. Haven't gotten around to posting about the other islands yet. Here's that link if you're interested. http://drfumblefinger.com/wrdprs/blog/category/aall/north-america/hawaii/
Enjoy your time on the islands!