Lanai is a small island about thirty miles from Oahu, the island that Honolulu is on. I live in Honolulu and on a clear day can see Lanai from the side of Oahu that is closest to it. It is only a 15 minute flight from Honolulu. You will have to fly in to Honolulu International Airport (HNL), then hop on a commuter flight to Lanai.
Lanai has only two five-star hotels (the ones I mentioned), and a small dumpy hotel called Hotel Lanai. There isn't much in the way of night life, the hotels are the only places where people are up after dark. They have their own bars and dancing. The days, however, are packed full of things to do. Sporting Clays, off-roading, snorkeling, golfing, horseback riding, etc. There is a ferry that has daily service between Lanai, Molokai, and Maui, so you can make day trips to those islands and return to Lanai for the night. Being on a boat while seeing those three islands is the best way to see them. Lanai, Molokai, and Maui form a triangle that is Maui County, so when you are on a boat going from one to the other, the views are stunning. I can't emphisize this enough: you and your partner MUST be outdoor-loving people, you must rise at first light, make full use of every day, and by night you will be too tired for any nightlife past 9 PM.
It will be the vacation of a lifetime.
One other thing: stay at the Lodge and go to the beach at Manele. There are vans that make regular trips between the two.