I did nearly the same itinerary, although adding in 2 nights in Mandalay a few Decembers ago. I thought timings were good apart from Ngapali beach. Despite the weather supposedly being perfect in December, I had 3 days of cloud and drizzle on the beach, which was quite miserable. Also, don't both snorkelling there as the reef is dissapointing.

Another vote for the Balloons over Bagan. Book early via a local agent, as they have better rates than booking direct. I stayed at the (Thiri Marlar Hotel ([email protected]) which I would reccommend.