6 days in Northern Vietnam

Hi there,
6 days does seem like quite a rush to fit all 4 spots in. Although you could do a day trip to Ninh Binh / Tam Coc from Hanoi, the basic Halong Bay tour is usually 2 days and the same for Sapa if you're planning on seeing some of the surrounding countryside. The town of Sapa itself isn't that exciting but the countryside is amazing. So if you're spending 2 days Halong, 2 in Sapa, 1 in Ninh Binh that only leaves 1 in Hanoi which isn't much time.

Thank you so much for your speedy reply. What can we do in Hanoi within 1 day?

Assuming you have a full day, you could explore the Old Quarter, Hoan Hiem Lake and its temple, Hoa Lo Prison, Temple of Literature and maybe the Military History Museum, but I'd really recommend 2 days in Hanoi personally.

Even two days is pushing it a bit for Ha Noi. In the past I have spent well over a week there and still find new things to see. I prefer the Ninh Binh province to Ha long Bay but that is because it is easier to explore on foot and the rides in the low boats through the caves is wonderful, just don't go there on weekends.
Please do not rush when in VN. The street scenes and the ambience of the country is every bit as memorable as the sights. And the chances are you will want to return.

Sorry, should have said at least 2 days in Hanoi. I loved just wandering around the city and even the 4 days I was there felt short.

Absolutely.
I once spent ten days there. Didn't get bored once. I loved the chaos of the OQ, and contrarily, the serenity of the parks, the Temple of Literature and the superb Ethnology Museum.

You do not have enough time for Sapa (At least 3 nights including 2 night trains). Ninh Binh or Mai Chau is an alternative to Sapa.

I'm in Hanoi now and have already been to Sapa. Will be going to Halong Bay soon. I would say that 6 days really isn't long enough at all. Hanoi has a lot to offer in itself. I wouldn't want only 1 day. If you only have 6 days, I'd skip either Halong or Sapa.

For Sapa we recommend at least three nights. You don't say when you are going so a little hard to give more advice. In the mountains it can be cool and wet.
here is a video I put together of our travels in N Vietnam.
https://vimeo.com/93068496