I found Hoi An and its surrounding area the highlight of Vietnam. It has a UNESCO-listed historic old town that miraculously avoided destruction in the war and even more miraculously has still largely avoided the over commercialisation of the modern day; its An Bang beach is beautiful and has a truly gorgeous bay setting; there are 500+ master tailors ready to make anything you want cheap as chips; and the other UNESCO-listed ancient Cham ruins of My Son within an hours drive
Make sure to give yourself at least 2 days, ideally 3 to really enjoy this place
And for ideas on how a trip to Hoi An can fit into an itinerary for experiencing the highlights of Vietnam, see this entry A 3 week itinerary for the highlights of Vietnam