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.
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.
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.
Halong Bay is one of the so-called new wonders of the world. The landscape of little hills and rocks in the water is indeed impressive. Enjoying this on a cruise ship for your honey moon is something that many people do.