I spent a few days in Hanoi in Feb 2006, but only really as a stop off point to do overnite junk trip on Halong Bay. We stayed in the old quarter (a rather cultural experience!) at a place called Hong Ngoc Hotel, but I wouldnt really recommend it. It was probably about 2 star and very basic. I cant even tell you the price cos it was all included in our fare. There are about 3 or 4 Hong Ngoc Hotels in Hanoi, all about the same standard. We werent there long enough to note some other better Hotels to stay at - sorry!
As for Halong Bay - I would highly recommend it, except when we went in Feb it was rather chilly and we actually had to buy a warm jacket at the wharf before getting on the boat! I havent been to Krabi so cant comment on if it is similar or not. The overnite junk trip was good and worth it. There were only 8 of us on the junk with about the same number of staff. We did sea canooeing and a cave walk. Food all included and really fresh and nice too. From pictures it looks similar to Thailand islands ... so I dont know if that helps you at all, cos I havent actually yet been to Thailand ... BUT ... only 50 more sleeps to go!!!!