I assume that you have also seen Hue before; otherwise, although I haven't been yet, that would be a much more interesting trip.
In Hoi an, walk to the covered bridge. There is an old house where for a small donation, they will let you see it; somewhat interesting. Let me see if I can find name of restaurant we ate at; very good. Go to marble mountain if you didn't go last year. Our guide took us to a silk factory where you can see how silk is made all the way from worms to fabric; fascinating. At the silk factory, they have silk "lanterns." If we had known we weren't going to see them anywhere else, we would have bought a lot more. If you are not going to Da Nang, I would definitely take the time to travel Hoi Van Pass on the return; it is quite scenic.
When I get home, I will try to see if I still have info on old house and restaurant.