Sapa and Halong Bay in Dec

We will be in the Hanoi region in the 1st week in december for just over a
week. We want to visit the sapa and halong bay regions - what are they going
to be like at this time of year? Also, whats the best way to see these
places - can they be done independantly or are we just better of with
arranging a tour from Hanoi.

We are comfortable travelling independantly but we have a return flight
booked from Hanoi which we can't miss and also not sure of the conditions.
If we arrange a tour - can anyone recommend a tour company?

Thanks