Are there any recommendationsof where to spend Christmas especially Christmas Eve and Christmas Day?
Do the restaurants in the park serve different food on Christmas Day? Or is this offered at hotels?
Is it best to get to a park early?
Are there different activities, parades etc on these days?
As a family we like to spend a few hours in them morning in a park and then return to the resort and maybe go to another park in the evening, is this a good idea during the Christmas period?
There are holiday snacks offered in EPCOT, and some of the restaurants have Christmas-specific menus or specials for Christmas Day. There's a list posted by WDWInfo from this past year
here. Many of the restaurants will have higher than normal pricing due to it being a peak time, so factor that into your food budget.
Get to the parks EARLY. Cannot stress that enough. We were there last year from Christmas to just before New Year's, and the crowds get increasingly worse the closer it gets to NYE. If you leave the parks during the day there is a chance that due to capacity closures you may not get back in. However, park hours are often extended late that week and crowds thin out a bit as people get exhausted and leave for the night, so taking an afternoon break and heading back to the parks early evening to close might work well. And if there is a morning EMH available, take advantage of it.
Be strategic with your Fastpass choices - we made afternoon picks for rides we KNEW we had to ride, and then hit up as many things as we could at rope drop before lines formed. There will be long lines everywhere as more people arrive. (To put it in perspective, Figment was at least a 45 minute wait every single day we were on property...which is insane.)
EPCOT typically has the Candlelight Processional, Holiday Storytellers, and Joyful for holiday specific entertainment. I don't think that AK has anything really specific to the holidays aside from basic decorations. DHS may or may not do Osborne Lights this year, but if they do, it's definitely worth seeing. Magic Kingdom will have their Christmas parade and Holiday Wishes fireworks.
Both MK and EPCOT do great NYE fireworks from what my cousins tell me (they're Florida AP holders), but anticipate not only a crush of people for the fireworks themselves, but also for trying to get out of the parks afterwards. (Your best bet would be to stay in a resort close to the park you plan on seeing fireworks at and walking back to your hotel, honestly. I've heard stories from friends where they waited hours to get out of the parking lot once the fireworks were done).
All the resorts and parks will have Christmas decor up, and many character meet and greets will have holiday costumes - we had pictures in AK with Minnie and Mickey in holiday garb, and Baloo and King Louie both had holiday scarves on. Some of the resorts have additional decor worth seeing - the Beach Club does a chocolate carousel, and the Grand Floridian does a giant gingerbread house, for example.
Despite all the warnings of crowds, it truly is a fun time to be at WDW, and as long as you put in a little planning the crowds shouldn't detract from the spectacle. My family enjoyed themselves immensely.