When I was there several years ago I did 2 nights Hue and 2 nights Hoi An and stopped in DaNang on the drive from Hue to Hoi An to see the Cham museum which was a nice break to the drive.
I liked Hue quite a bit. The imperial tombs are quite interesting and the citadel is big. Weather was not great for my time in Hoi An but it was worth spending a day walking around the town. I stayed at the old Art Deco hotel in Hue (can't recall the same off the top of my head) which was very nice.