I spent about 9 or 10 days in north and central Vietnam a few years ago, so your 18 days I think will be plenty leisurely.

You may want to look into Sapa as well. I did not go there myself.

But I did Hanoi, Halong Bay, a couple of day trips from Hanoi, then Hue and Hoi An. I liked Hue (even though I was sick one of the days there). Hoi An was worth a wander around. There are some beach resorts that are pretty nice in that area. I happened to be there in November so I had one OK day and one rainy day.

I liked the Cham history museum in Da Nang.

I liked La Residence Hue when I stayed there. I think it was the nicest hotel in town. Some cool art deco touches if you are into that. The citadel is interesting and the emporers' tombs are fascinating.

I really enjoyed my day trip to Perfume Pagoda when I was in Hanoi. You take a row boat up this shallow river and then get to this little area where they have a bunch of places to eat. You can either climb up the mountain or take the cable car. My guide said he hated taking people there because he didn't like the people there. But it was one of the places I liked best on my trip.