World Heritage Listed Halong Bay, an utterly memorable part of your trip, overnight is the only way to go. Don't make the mistake of even contemplating rushing this as a dodgey $100 day trip from Hanoi. As part of my recent Intrepid Real Food Adventure in Vietnam we spent a night cruising Halong Bay on a Bien Ngoc vessel and enjoying some of the many extras as part of the destination. They are a great comfortable vessel. Each cabin has twin beds, shower, toilet, air conditioning, lights, power points and a fan. All the necessities you need for an overnight or 2-3 night cruise.
It is at least 3.5 hours drive to Halong Bay from the centre of Hanoi City so I advise that you don't try and squeeze Halong Bay into one day. To really enjoy and savour in the best that this region has to offer you need to do at least a one night cruise. But be aware that paying top dollar doesn't necessarily mean you will be away from the crowds. You need to take advice from your booking agent to help make sure your adventure to Halong Bay is memorable for the right, rather than the wrong, reasons. There are some very basic/tired vessels out there so be careful. Getting to Halong Bay around midday you will check in with your boat crew, get assigned your cabin and usually within 30 minutes you will be heading out of the harbour area towards the islands. We had the pleasure of being served a gorgeous local seafood lunch while we sailed out to the islands. Gliding between towering limestone karsts and marvelling at mother natures creations.
Most decent overnight packages will include swimming at one of the beaches (swimming from the boats in the bay is banned by local authorities), kayaking around the bay in designated areas or an Island cave walk. Some of these caves have only been discovered in the last 100 years and there are roped off pathways through the interior which stops the limestone formations from being damaged. Every turn is another WOW moment and you really must take your time here to soak it all in. Not to mention they are much cooler than the intense heat outside.
Enjoy delicious feast after delicious feast exquistely prepared by an onboard chef. Just partaking in Vietnamese food is the best detox you can get. Fresh produce, prepared in the most basic but stunning way and flavour that is often forgotten in the western world.
The evenings are still in the bay, many Chinese travellers (on their own larger boats) will be heard doing Karaoke well into the night. We took the opportunity to sit up on the top deck, enjoy a few cocktails, photograph the lights of the night and marvel at our surroundings.