Slackjawed:
My wife and I just returned from Lana'i and the Big Island a couple of weeks ago. It was our first trip to Lana'i. We stayed at Manele Bay. We ate at the Hotel Lana'i which was excellent (but not cheap!) The two Starwood hotels -- Manele Bay and The Lodge at Kehole (sp.?) have a shuttle that runs between them and stops at the Hotel Lana'i. There's a shuttle every twenty or thirty minutes. You could definitely get away with not renting a car. I am sure that you could take the Starwood shuttle (they would never know that you weren't staying at one of their hotels), and if you are going to one of their hotels to, among other things, eat, I'm sure they wouldn't be upset that you are taking the shuttle.
Lana'i is very nice. It's definitely worth going to. We spent 2 1/2 days there. I think 3 to 3 1/2 days is plenty. You'd be really bored after that. It's a very peaceful island but aside from some hikes and snorkling (and the obvious lounging by the beach/golfing), there's not a whole lot to do--not that there's anything wrong with that.
If you go, you definitely should check out the 2 other hotels. They are really beautiful and worth seeing. The Lodge is maybe a 1/4 to 1/2 mile walk from the Hotel Lana'i. It has this amazing putting golf course--like putt-putt but real greens. It's fun for those of us who don't golf much or ever.
We took a snorkling trip that took us past the huge cliffs on the south/west end of the island. The cliffs are huge--soaring straight up 1100 feet from the water. The snorkling was pretty good but not phenomenal. We saw a ton of dolphins up close and a bunch of whales from a distance (it was whale season).
I definitely recommend Lana'i. I can't speak to the quality of the Hotel Lana'i but it seemed okay and it's restaurant was great. Lana'i has a real small town feel to it--only 2700 residents and everybody knows each other. It's well worth a trip.