If you're visiting Hanoi you'll likely be staying in the Old Quarter.
When I lived in Hanoi I would always take visitors to see the old French Cathedral, St. Joes, to start the day.
Just across the street there is a great little secret coffee shop called Hanoi House. In the evening it turns into a cocktail bar. It's got an outdoor balcony that looks across to the church. There's so many cool spots hidden in the alleyways of Hanoi that most people would never discover on their own.