Ha Long Bay, Cat Ba or Ninh Binh — which should you choose?
If you only have one extra day from Hanoi, choose Ninh Binh. Choose Ha Long or Lan Ha Bay if spending a night on the water is something you genuinely want to do. Cat Ba is a good option if you want more independence and an island base rather than a standard cruise.
Answer
Ha Long Bay and Ninh Binh both have dramatic limestone scenery, but the experience is quite different. Ninh Binh is the easier option from Hanoi. You travel through rivers, caves, rice fields and villages, and you can see a lot in a day. Staying overnight is better if you want a slower pace, but it does not require the same commitment as a bay cruise. Ha Long and Lan Ha Bay are more about the experience of being on the water. An overnight cruise normally takes up most of two days once the transfers are included. I would choose it if sleeping on the bay, eating on the boat and waking up among the islands sounds like part of the trip you want. If that does not particularly appeal, Ninh Binh is usually the better use of limited time. Cat Ba gives you another way to see the bay. You can stay on the island, arrange boat trips from there and have more freedom than you would on a fixed cruise itinerary. It makes sense if you like the idea of an island base, but it is still part of the Ha Long/Lan Ha experience rather than something completely different. If you are also going to Sapa or Ha Giang, I would normally choose either Ninh Binh or the bay. If you are not heading into the mountains, doing both is much easier. Just keep some flexibility around the bay: bad weather can affect sailings, so check conditions close to departure.