Ninh Binh gets sold as Ha Long Bay on land, and the nickname is only half right. There's no sea, and the cliffs are lower. What's real is the shape of the place: a block of limestone where the valleys between the peaks are flooded, so you travel through the mountains by rowing boat, ducking into caves and coming out in rice fields. The area is a UNESCO World Heritage Site, listed in 2014 for both nature and culture, because it also holds Hoa Lu, the capital of Vietnam for about forty years in the 10th century, before the court moved to what is now Hanoi.
It's a small place with big sights. Trang An, Tam Coc, Mua Cave and Hoa Lu all sit inside a circle about 15 km across, and the town of Ninh Binh itself has almost nothing to see. Three days means two nights, and that's the argument for sleeping here instead of joining a Hanoi day tour: the tours spend about four hours on the road and arrive when the boat queues are longest. Stay, and you can climb Mua Cave at opening time, ride the boats at the quiet ends of the day and cycle the lanes at sunset.
The plan keeps each day in one corner: Trang An and Hoa Lu, then Tam Coc and Mua Cave, then the quiet north with Van Long and the giant Bai Dinh temple. Start on a Monday if you can, because Vietnamese weekends fill the boats and raise guesthouse prices. Most people arrive from Hanoi, 100 km to the north, and the 5-day Hanoi guide covers the capital. For the wider picture, see Vietnam.