51 To Gian Alley, Phuc Tan
Phuc Tan bar, formerly The Lighthouse, is a big nightclub with a huge area for dancing, along with a patio out back with views of the river and Chuong Duong bridge.
It's crowded, dirty and the toilets are rated as the worst in Hanoi but it's a popular spot for late night drinks and a dance, staying open until well into the early hours, and attracts some decent DJs.
Barbecue food is available outside for those late night munchies. Entry is free except for on rare special events when the hefty bouncers won't let you past without a stamp.
It's across the highway from Hoan Kiem district. From bia hoi corner, head east on Luong Ngoc Quyen St to the highway, carefully crossing and heading north to Thanh Yen St. Follow this past Phuc Tan St to Tu Gian alley. It's only a 10-minute walk but it's devilishly hard to find, and crossing the highway on foot is treacherous, so hire a taxi to get here -- but be warned, taxi drivers may not know where it is, so take a map with it marked on.