We have always had very good luck going to Epcot on Sundays. It really depends on the time of year but we have run into surprisingly small and huge crowds at MK on Sundays. If you have park hoppers you can start out in MK to get your fix and then take the monorail over to Epcot if it starts getting crowded.
We will be there that same time. We are actually planning a CRT breakfast first thing Sunday morning, so we can hit Fantasyland and a few other big attractions before the crowds come in, then I think we're heading to Epcot later in the day.
Uhh, maybe Central Park? Seriously, you know there are going to be big crowds and you've just got to be prepared to deal with it. Wherever you decide to go, get there early, and just go with the flow. Don't stress over the crowds cause they're beyond your control. Have fun!