For some reason Ha Long Bay has always reminded me of some James Bond movie (perhaps the Golden Gun, any Bond fans out there..?) so I had to look into it just now and according to my quick search they filmed in Thailand but pretended to be in Vietnam. So I guess there is some connection there.
We went on a day cruise around Ha Long Bay. You can choose over night ones as well and as I understood it you will go further along on the longer cruises. The scenery was beautiful and there was something happening all the time to keep you busy. Like small boats coming up selling fruits or other refreshments, you could go kayaking and we also went for a little hike through an island. But if you just wanted to relax you could lay down on the deck and enjoy the ride and sun!