Well at first, you have to take a bus from Danang to Hanoi is gonna be a long way from Danang by bus or train. From Hanoi there are plenty of travel agency to choice to book your trip to Halong Bay. There are plenty of things to do while you’re in Hanoi such as; Ninh-Binh, Tam-coc,and Sapa.There are a lot of tips here in VT about Vietnam you can check them and try to decide which of those you’d like to see.
There is no boat sailing from Danang to Halong! If you have time, you can hop on / hop off any open bus from Mai Linh Bus and Sinh Cafe from Danang to Hanoi. You also can book directly to the Boat in Halong which have their office in Hanoi with round transfers from Hanoi to Halong.
Danang-Halong = nearly 900 km, with a lot of things to visit in between (not counting Hoi An 35 km south of Danang). Main site to visit : HUE (2 days is OK) ; then night train Hue-NINH BINH to visit the magic Halong Bay on Land of Ninh Binh/Tam Coc (2 days in Tam Coc ; lots to see !!!!) ; then public bus Tam Coc-Hanoi ; 2 days in Hanoi ; then cruise Halong (2 days are OK ; 1 day no good because it takes 8 hours to go RT Hanoi-Halong). Many agencies, from appalling to excellent. If you not afford a private junk (the best, but not cheap), their Budget Collective junks are impeccable. Check also their visa service for the visa on arrival, much cheaper than through a consulate (attention ; visa on arrival only if you arrive in Vietnam with an INTERNATIONAL flght, not if you arrive by land).