I just booked a trip to Thailand in late December/early January. Have about the same number of beach days - here is how I decided to break out my days:

Day 1 (after 5 days in BKK including New Year's Eve) -- BKK-HKT around noon, drive to Krabi
Day 2 -- Krabi
Day 3 -- Krabi
Day 4 -- Drive from Krabi back to Phuket
Day 5 -- Phuket
Day 6 -- Phuket
Day 7 -- Phuket
Day 9 -- Phuket
Day 10 -- start trip home around 5 PM (HKT-LAX with a few stops)

We will do a day trip to Phi Phi one of the Phuket days. Would stay a night over there, but will have an infant with us, so we are more point to point than normal (trying to minimize the number of times we pack/unpack). Have booked the Sheraton Krabi Beach and Le Meridien Khao Lak using SPG points.