Ill keep this short because I have so much good stuff to say about home.
generally Hawaii is much slower then the US mainland. Oahu is the most heavily populated of all the islands. Oahu is also the busiest Island of them all. Maui would be the next busiest island but definitely slower than Oahu. every island has there scenic views and tourist hotspots so regardless which island you choose to visit you will not be disappointed. If you want a slower pace more kick back environment I say kauai, maui, or, the big island. Little more up beat, lively nite life, and, shopping, I say Oahu. Every island will have their own cool places to visit.
Do some quick research on each island and see which one has the most things that interests you. Perhaps you can island hop.
We can "talk story" as us locals say. Ill be happy to answer any questions you may have.
Leave a message if I don't answer. I'm a busy guy.
Oh and btw congratulations to you and your lady:)