Sapa at the moment is a big building site (was there a week or so ago) – the company which operates the new cable car that runs to the top of Mount Fansipan is building a huge new hotel/complex in the centre of town – this is going to completely dominate the place. The market which used be be just off the main street has been moved to the edge of town near to the bus station – might be more convenient for the traders and tourist coaches, but has lost its character and has left a big void in the heart of the town. Still worth a visit, but changing rapidly.
Hanoi – very easy to while away several days there drinking Bia Hoi and soaking up the atmosphere. Agree with comments about choosing Cat Ba rather than Halong Bay. Hoi An old town is nice, but the nearby resorts are characterless. Wasn’t particularly enamored with HCM – much prefer Hanoi.
If you want to avoid the hassle of organising everything yourself then look up a Hanoi-based tour company called More Fun Travel – very professional and 101% reliable.