Ha Long Bay is a Unesco World Heritage Site and is definitely not to be missed if you are in Northern Vietnam. Reached easily from Hanoi, the bay is a beautiful spectacle due to its turquoise waters and thousands of limestone karsts that dot the lake. The karsts create caves, tunnels, lakes and lagoons that can take days to explore. Take a junk (large boat) from the port and they will bring you to a floating village, a cave and a place where you can go kayaking. If you feel so inclined, spend the night on an island and continue deeper into the bay for the next 2,3 or 4 days.
Do not expect to swim because the bay's waters are polluted at the moment, but they are in the process of cleaning and conserving. The junks can be either booked from Hanoi or from the port directly.