Six days is not long for a country this shape, so this programme flies rather than drives between regions. You land in Da Nang, spend the first afternoon in Hoi An Ancient Town, then give a full day to Ba Na Hills and the Golden Bridge before the late flight north.
The middle of the trip is Ha Long Bay: on board by midday, lunch as the boat sails past Dog Rock and Thumb Rock, Titop Island in the afternoon, kayaking at Luon Cave, a sunset party and a cooking class, then Sung Sot Cave and tai chi on the sundeck the next morning.
Back on land there is a half day in Hanoi, from Tran Quoc Pagoda to Hoan Kiem Lake and a cyclo ride through the Old Quarter, and a last full day in Ninh Binh at Hoa Lu, Tam Coc and the climb at Mua Cave.