I'd certainly have as much present shopping and wrapping done in advance as I could. But there's still a good few days if mid-December. Certainly anything that needs to be sent by post I'd have wrapped and packaged for post office to send immediately on return (if not before I go). Anything that you think you'd have to drop off that single weekend in between, I'd also try to have wrapped and packed in bags to grab (as in, all presents for 1 household put together and ready to just pick up as you walk out the door - and 1 bag (or more if needed) per household involved).
Cards - you are back early enough to still send all but international ones. So I would suggest bringing them to write on the plane (my usual cunning plan when going to meetings abroad from September!!), along with stamps so you can just throw them into any post box you see once you get home, even in the airport on the way home. Any international ones? Perhaps any for the US write on the outbound flight and post while there. Any for other places - check latest postage dates in advance, including from the US, and see what suits best for those. (Also, another cunning plan could be to bring any US/Canadian packages you may have, and post those locally too).
Book online food slot is very good advice. I'd be inclined to book 1 for the day after you return, for general shopping for that week and non-perishables, and another after the weekend for the rest.
Certainly check the decorations and lights before you go. But if you normally have a real tree, organize with your local supplier to keep you one that suits (you may need to pre-pay) and get that when you get home to decorate over the weekend. If you are not normally a real tree family, yes, get the fake out and out it up in advance (could have 1 set of lights on a timer as part of your "making house look occupied while away" strategy).
If DCs are of an age where Santa is important, I definitely think I'd see him in Florida. If you are there mid-week, shouldn't be a problem. And it may be more magical as well.
Make sure to get to the Christmas shop in Disney (I think it's in Epcot, German part, but I could be wrong about that - open all year though) and get a tree decoration to remind you next year! Although I am sure they'll have lots of decorations in ALL the Disney stores at that time of year.
Enjoy - I am so jealous!!