On the return, Friday morning isn't bad at all, with each successive day getting a little busier, more difficult and more expensive through Sunday evening. The bottom line: If you are looking for a deal, you won't find one on the peak travel days. Travel off-peak whenever possible.
With both Christmas and New Year's falling on Sunday this year, the Friday before and the Monday after each holiday will be truly gnarly travel days, as travel will be concentrated on the bookends of a single long weekend in a way that would not occur when the holidays are mid-week.
Your best bets are reserve your Orlando vacation rental as early as possible in the week before the Christmas holiday, particularly from the 19th to the 21st, and as late as possible in the week following the New Year's holiday. In the week between Christmas and New Year's, traveling midweek is your best bet, with Wednesday the 28th shaping up as the best of the lot.