Hoi An is a nice little city with lots of things to see and do for tourists, including art centers, river rides, lampoons and candle light, beach front and great food.
Da Nang is the 3rd largest city in Vietnam. It is located within 100 km of several UNESCO World Heritage Sites, including the Imperial City of Hue, the Old Town of Hoi An, and the My Son ruins. Da Nang itself has a long beach with many luxury resorts.
Hanoi is a busy city with a great market place. All kinds of stuff is sold in small stores. As in the whole of Vietnam there are many scooters, but not as many cars... Experienced travelers make Hanoi a must-see city in South-East Asia.
Ho Chi Minh City is probably better known as "Saigon". It is the 2nd largest city in Vietnam, but not very touristy. The few sites are mostly related to the Vietnam war with the USA, for example the war museum and the deep, narrow holes / channels, where the Vietcong hid themselves and fought.
Hue has a few UNESCO heritage sites. The Citadel may still need some renovation work, but the nearby Tombs of former kings are worth a visit. The night market along the Perfume river is also very pleasant.
Sapa is a quiet but touristy place in the mountains. Interesting tribe villages and rice fields around. You can get there by train from Hanoi. Note the weekly Bachma market not too far from Sapa where locals still wear their traditional dresses.