I've been focusing on the Florida Keys after getting advice on the thread.
We fly from Seattle and would like to use Alaska to use some credits up. What is everyone's preferred airport to fly into?
Since you are not too particular, I definitely suggest pricing out different options and see where you can find a great air+hotel rate. Also make sure to look at recent reviews, given the amount of storm damage last year. Current ocean temps are also very high, which often portends an active hurricane season, so none of us really knows what might happen in the months ahead. Hopefully not much in the way of storms, but it is too early to know.
It is hard to say exactly what type of weather you'll get, because December weather can be a bit unpredictable. The northern FL beaches tend to be somewhat chilly. The southern FL beaches might be in the low 80's. That's what most folks think of when they think beach weather, but might be warmer than you prefer.
The Atlantic side of FL has somewhat bigger waves. The Gulf beaches tend to be pretty flat. a plus of the gulf side beaches is that they tend to have lots of shells, so even if it isn't swim weather, it might be fun to go shell hunting.
It also bears mentioning that FL has lots of beaches. If the beach you choose isn't quite to your liking, drive a little ways, and you can probably find another beach that has a different personality. Some beaches are more built up, others are more nature park. They Keys are maybe one exception, because they are a bit isolated. Still, driving out there is quite something.
Another fun option in south FL is to see the Everglades and Big Cypress. You can see some of it by car if you wish. Taking an airboat tour is also pretty neat. Winter is a good time to see gators and manatees. There are many great places in FL to see manatees. If you have children, and visit a national park, they can do the Junior Rangers program for free. Usually = a free booklet with activity pages. They fill it out, and get a badge after a short ceremony.
If the weather isn't ideal for sunbathing: Tampa has a bunch of fun options beyond beaches. St Augustine has all kinds of very neat history to explore. Everyone should see the Fort and walk the Old Town area. There are a number of zoos and wildlife places near Fort Lauderdale and Tampa. Ft L offers the Jungle Queen boat tour that gives you a glimpse of how the uber wealthy live, and a tour of the city. Also, consider Florida State parks. Silver Springs is neat to see, but it is inland and about 1.5 hours north of WDW.
IMO, most airports are in many ways all the same. I would choose so much on 'best airport' as much as best flight options/lowest price. In December try to get as direct a flight as possible. Also know, winter flights always = a fair risk of getting stuck to do big storms moving across the country. Always allow an emergency fund in case you get stuck in FL for an extra day.
