Try to stay in the Old Quarter, stayed in a hostel there last year. Great area imo.
http://vietnamhotels.net/travelguide...rter-in-hanoi/
Oh and if possible try to do the trip to Halong Bay and do an overnight trip. Waking up in the morning in this scenery is amazing.
http://en.wikipedia.org/wiki/H%E1%BA%A1_Long_Bay
