I'm planning our first trip and we are planning to go to MNSSHP but I received some great advice on here. It was recommended we not do MK the day we're doing MNSSHP b/c we'd be paying twice to get in. So, for us, we'll be hitting TL or DQ that day, breaking for dinner and then off to MK for the party. If you don't want to go to the party I would recommend Epcot or MGM - you can check park hours on this site. Also, if you subscribed to touringguides.com you can see what park is recommended for that day and which parks they recommend you avoid due to crowds and such.
Good luck and have a great trip!