Could someone please explain the comments about not going anywhere the first half day so that a 'day' pass isn't wasted. We always stay on site and our tickets are always what my travel agent calls 'length of stay passes', valid from the time we check in until midnight of the day we check out. I just assumed that everyone who stays on site has the same type of ticket.
Well, I'm not sure when the last time to went to WDW was, and there might still be length-of-stay passes, I don't know. But it seems that, for the most part, Disney is pushing the Magic Your Way passes instead. You start with a base ticket, & choose the amount of days you want on your pass. It allows you entry to one park a day, and expires 14 days after you first use it. The more days you add-on, the cheaper it is per-day, to the point where there's only a 2 dollar or so difference between a 6-day pass and a seven-day pass.
From there you can add on options like Park-hopping, access to water parks and Pleasure Island, and No Expiration. A lot of people, at the very least, pay for the No Expiration option, which is why so many people are talking about "not wasting a day." This way, if they don't use all their days, they've got some days for future use, no matter when they go back, years or even decades from now.
So I guess the big question we should ask the OP is, what kind of tickets do you have? Would you be conceivably "wasting a day" if you get to a park in the mid-to-late afternoon?
If you have options galore, I'd say go for a water park, hit the slides if you have the energy, soak in the lazy rivers if you don't. They close early, have dinner at your hotel, explore the grounds a little, then get up nice and early the next day for MK.
If you gotta hit a park, I'd say, first see if you can reservations for Rose & Crown in Epcot's England Pavilion around 8 o clock. If you can, got to Epcot. Ask to sit outside if at all possible for dinner, so, even if you're excited, you'll have a nice place to sit and relax during Illuminations.
If you can't get R & C, & if you have a rental car, I'd say try to get ressies for Boma at AKL, and go to AK, which closes early, and then have dinner at Boma.
No matter what, enjoy yourself!