Da Nang is great for a beach holiday, sand and sea very nice. Plenty of bars and restaurants. But worthless for sight seeing and shopping.
Hoi An is a tourist trap to avoid, packed with Chinese tourists, and souvenir shops.
Hanoi is the best sight seeing ever.
Saigon is great for beer, hookers and shopping. Not much to see though, but the fast ferry to Vung Tau for a day at the seaside is nice enough.