I haven't been at Christmas, but I can address your safety concerns. That was one of the things I LOVED about Ambergris Caye - it is safe. San Pedro is a very laid back and safe community. The people are friendly, and the vendors are not aggressive.
My husband went to the ruins at Lamanai (I was a bit ill that day), and LOVED it. I plan to go this year.
Last year, one of our favorite dives was an all day trip to Blue Hole/Half Moon Caye/Aquarium dive. Since I had just been certified, I snorkeled the Blue Hole - still SO much to see. The wall is full of life - saw a gorgeous green moray swimming along side it, and a turtle kept us company during our safety stop. Lunch on Half Moon Caye is amazing as well. Take your camera!!!!
I HIGHLY recommend booking dives through Amigos Del Mar http://amigosdive.com
. The crew is wonderful!!! All of our dives were booked through them and I have nothing but fantastic things to say about them. We plan to do the Turneffe dive (among all others) with them this year (11/5 - 11/15).
I'll post when return and let you know how it went.