We do lots of adult only trips and these are the things we enjoy:
Dinner at Victoria & Alberts. If that's too much for you, book one of the 3 upscale restaurants at the Swan/Dolphin. They are much more adult oriented than any of Disney's signatures and they have better food.
Tours. Since most of the tours have an age limit, it's fun to do these. My personal favorites are 2 of the 3 Segway Tours. Segway Around the World goes through the World Showcase and the Wilderness Segway is an offroad tour around Ft Wilderness. I'd also love to try the new tour at Animal Kingdom.
Spa treatments are always good.
La Nouba at DTD, although a family show, is fun as adults only.
Jellyrolls on the Boardwalk is fantastic. Especially if you walk there after having dinner at the Swan/Dolphin.
Since you are staying at OKW, my favorite resort for relaxation, I'd do some of that too. I love sitting on the balcony at OKW for coffee and book reading in the morning.
Rent a couple of beach chairs at Typhoon Lagoon and spend a day relaxing there.