I ended up taking two trips out to the bay because I loved it so much, but fortunately both were different experiences and with different people; the first trip was for 3 days/2 nights with Matt; the second was just for a day trip with my parents.
Halong Bay is about a four-hour bus ride east of Hanoi, which makes it a busy day but is doable - I recommend at least two so you can go further into the bay and away from all the tourists and boats.
You arrive at the bay with thousands of others at the docks, you grab a ticket, life-jacket and then board a water taxi to take you to your ‘cruiser’ - the boat that is going to take you around the bay, and sleep in - and you set off into the bay.
After an hour or so you make some stops on some of the karst islands to visit some caves.
Cruise around the bay in a smaller boat that can explore all the nooks and crannies.
Kayak amongst the limestone pillars.
And for us, we spent our first night aboard the ship, and the second in a private island beach house; worth the upgrade!
Followed by a day heading out to visit the local communities living on some of the small islands deep into the bay.
Halong bay is truly magnificent. Spending a few days in the bay gives you the benefit of taking it easy and getting away from the tourists, but most of the activities involve lots of walking, cycling, swimming, kayaking and other physical activities (unless you got for a luxury cruise). My parents found that a day was long enough to see the sights, but I think spending at least one night in the bay is something quite cool.