we've vacationed in Key west a couple of times and had a great time although the time we did New Year's, it was the coldest on record.

We had several layers on as we had flown in from a spending a week in California visiting family so we didn't have "winter" clothes. I think we stayed at Casa Marina-very nice hotel. When we got into our room, we could see our breath! Hubby called down to the front desk and said that something was wrong with the thermostat as he couldn't get the heat to come on--they don't have heat! lol
We rented just about everything you could as far as transportation, lol. The electric cars, a real rental car, motor scooters and then we traveled on the buses. We had a blast as it was our first vacation in about 5 years.
We've had a blast everytime we've been. You'll have a great time.