Is northern Vietnam enough for a first trip?
If this is your first time in Vietnam, you can stay entirely in the north and still have a varied trip. With 5–6 days, spend time in Hanoi and add one place such as Ninh Binh, Ha Long Bay or Sapa. With a week, two stops can work. Trying to fit Ninh Binh, the bay and the mountains into the same short trip usually means too much time on the road.
Answer
Northern Vietnam has enough variety that you do not need to race south just to feel you have “seen Vietnam”. Hanoi gives you the food, street life and city energy; Ninh Binh adds rivers, limestone cliffs and countryside; Ha Long or Lan Ha Bay gives you time on the water; and the mountains bring a completely different pace and landscape. The catch is distance. Ninh Binh is easy to add from Hanoi. An overnight bay cruise takes up most of two days once you include the journey there and back. Sapa is a longer move, and Ha Giang needs several days if you want to do it properly. With 5–6 days, I would choose Hanoi and one other place. With a week, add a second. Ninh Binh and the bay are the easiest combination if you want variety without spending too much of the trip travelling. If the mountains are the main draw, choose between Sapa and Ha Giang rather than trying to do both. Sapa is easier to fit into a shorter trip. Ha Giang is better when the road journey itself is one of the reasons you are coming north. With around 9–10 days, you have much more room to combine Hanoi, the mountains and one or both of the karst areas. Even then, I would resist adding stops just because they are famous. Northern Vietnam is much better when you have time to stay somewhere long enough to enjoy it.