Research, research and then research some more.
Disney is an amazing place to holiday and we've never had a bad experience but we know what to expect because I'm rather an over/obsessive planner-guilty but sneakily proud!!
Decide which parks you want to do on which days - the December calendar will come out in the next few weeks. There are websites that will recommend which days are best to visit certain parks based on their own formulas. You could look at some of those for ideas. Once you've decided that, you can look at restaurants because those ADRs are the first solid plans you can make at 180 days. Resort guests can make them 180 days plus the length of their stay up to 10 days. You are going at an extremely busy time - 21st to 28th December. If you have an ADR that you really want be online at 6am EST 180 days out.
If you are interested in meeting characters then try character dining. It's a bit more expensive but you get to meet upto 5 characters without waiting - well you wait for them to come round but you're eating while you wait so that's all good!! We really enjoy Crystal Palace at MK (Mickey, Winnie-the-Pooh and friends) and Tusker House at AK(Donald, Daisy, Minnie, Mickey and Goofy.) We prefer the breakfasts and have 8am ADRS for both for our October trip so we can eat and be out for park opening. Being at the park for opening can be a really great way to get a lot of attractions done before queues start to build as more and more people arrive. Look up the menus for restaurants that catch your eye and cost what you would spend if you ate there then look to see if it's worth the DDP. We get it free as part of our UK package but if you're paying for it you want to know that you're getting your money's worth.
For quick service meals we love Flametree BBQ at AK and Columbia Harbour House at MK. We love the cupcakes at Starring Rolls in HS (look out for the snacks throughout WDW, not just cupcakes, they are amazing. I asked for help here in compiling a list of must tries and I am determined to plough through them all!) and there is so much choice at Epcot, we like Sushine Seasons for qs in Future World because it has a wide selection that suits the varying tastes of our family. A must do sit down dinner, for us, is Ohana at the Poly - great food, great service and great entertainment. There are so many, actually, depending on what you like and how much you want to spend.
Familiarise yourself with the park layout and have an idea of which attractions you want to cover and how best to do that. Use your 3 FP+ selections wisely - that's a huge change since your were last at WDW and a change you really want to be familiar with. Just about every ride has FP+ now and resort guests can make their selections at 60 days out. Some parks - Epcot and HS - tier their FP+ to cope with demand. Some attractions are best done with FP+ (the ones that have high wait times throughout the day) and others just aren't (any time attractions.)
When your 3 FP+ have been used (or the time slot is up) you can make more but this has to be done at a kiosk in the park where you want to use the FP+. Availability of additional FP+ is ?? It could be tricky to get good additional FP+ over the holidays but always worth a try?
Biggest tip - get to the parks early for rope drop so that you can be through bag check and be at the turnstiles for opening to take advantage of lower early morning crowds, especially as it will be busy.
Other biggest tip - Enjoy! Take time to soak up the atmosphere.