Halong Bay
Three hours east of Hanoi by road, Halong Bay is the limestone-karst seascape that ends up on every Vietnam postcard — nearly two thousand islands rising vertically from a green-gold sea. The standard visit is a one- or two-night cruise on a junk-style boat: kayaking through cave passages, swimming in calm coves, watching the sun rise from the deck. The crowds are real; for fewer boats and the same geology, try Lan Ha Bay (just south, accessed from Cat Ba Island) or Bai Tu Long Bay (just north).