Haiphong's one of the most easily accessed places in the country with regular air, rail links and good roads from Hanoi.
Haiphong's Cat Bi Airport is a busy domestic terminal and can be a good option for accessing Cat Ba Island (45 mins by speed boat from Haiphong town in the southern part of Halong Bay) or to head to the bay proper. There are flights from Saigon and other points. Both Vietjet, Vietnam Airlines, Bamboo Airway.... operate flights on the route.
The rail link (around 3 times per day) from Hanoi is great for train buffs. It's nothing flash and pretty slow (2-3 hours to cover 100kms). The slow run across the Long Bien bridge and the Red River at the start of the journey is a highlight.
A new highway connection makes Haiphong just 1.5 hours from Hanoi.