my family went the day after christmas once for vacation & it was very busy/crowded. one cool thing was the parks were open late, we were at magic kingdom until 2am. i would make your dining reservations now & any other activities ie the spa, tours, etc. in advance. also look into some of the special events like the candle light processional, mickeys very merry christmas party, osbourne lights & the special holiday fireworks. additional advise be patient & willing to change plans if the park becomes to crowded head to another park, back to your resort to relax, swim, etc.
have fun!