My hotel recommendation for a stay in Hanoi is:
The Sofitel Metropole: This would be the only hotel I would recommend staying in. It is the old French colonial hotel from 1901, with wonderful charm and amenities. The location is perfect from which to explore the city and Old Quarter. I loved staying in the older section that still has its colonial charm. Today it has seven bars and restaurants.
Engineers renovating a found a sealed passageway leading to an underground bunker. The bomb shelter under the hotel was built sometime in the 1960s to protect guests during the Vietnam War. The bunker was reopened in 2012. Hotel guests can take part in a guided tour.