We enjoyed Halong Bay by opting for a $15 all inclusive boat ride that did not disappoint. The day involved kayaking for about an hour, optional rock-climbing on Monkey Island and jumping off the boat into Halong Bay. Lunch was provided on the boat and had vegetarian options in the form of tapas for the whole table. The boat holds about 30 people and you can buy beer onboard.
Cat Ba Island
During the Vietnam War this cave in the side of a mountain on the island was used as a hospital. Invisible to the enemy, the Hospital Cave had numerous rooms, a cinema and even a swimming pool.