4 hours drive from Hanoi is the U.N.E.S.C.O. World Heritage listed site of Halong Bay. 1,500 klm square populated by almost 2,000 islets. Big limestone “mountains” that protrude from the water. The caves at Halong Bay are a must see, as is the Bay itself, even if commercialised and a popular tourist attraction. There are quieter places but that would entail a longer than 1 day trip from Hanoi. Beautiful but often dulled by the pollution from the nearby coal fired power plant at Uong Bi.