Remember that your MNSSHP ticket is your admission to the park as the party starts. If you spend the day relaxing at the resorts or shopping at DTD, you will not need to use a day on your ticket to go to the party.
This can be good or bad. If you are staying off property or are buying tickets day by day, you can save a bit of money by spending the day out of the parks and only using your MNSSHP ticket for entry that night. If you are buying a MYW multiday ticket or have booked your tickets as part of a package, you will have a one day ticket that will sit unused for each person in your party. With the MYW pricing structure, it is practically cheap to add just one day when you're buying at least three days. If you are staying for five days, either buy a four day ticket if you don't plan on going to a park that day, or live with the knowledge that you 'wasted' a day of your tickets.
I personally would recommend doing a light morning at one of the other parks, AK or MGM come to mind because you can go to either of those parks for three or four hours and get a few rides or shows done in that time, no problem. Then head back to the resort for lunch, a swim, and a nap. You could be snoozing by 2pm and wake up around 5pm to get ready to head to the MK for the party.