Family vacation at Christmas-6 adults. What are some fun activities
So I was born in Denver and lived in CO until I moved to FL to finish High School and University. I've been to Key West 3 times in my life.
First how to you plan on getting down there? If you drive (you probably will from Miami) look into spending a night in Key Largo. Go Scuba Diving or Snorkling with one of the Charters there. If you need more info let me know.
I've been to Key West Twice during Xmas. So there is plenty to do there. We stayed both times at the Sheraton Suites Key West it's a little far from downtown, but well worth it. It's a nice little resort.
The Main strip is full of well known bars ( Sloppy Joe's Bar, Hog's Breath Saloon, etc) so that should keep you busy. A day at the Ernest Hemingway Home & Museum is nice as well as maybe a boat tour or a day at the beach. If you are feeling very adventurous might want to look into the Dry Tortugas National Park.
Hope this is a nice start, but let me also know what types of things you like to do.
Mentioned in this answer:
We love going on the Conch Tour Train. It goes around the city and tells you about the history of the city.