EPCOT is my choice for adults as a first option. Any of the other parks that you would like to do alone, would be my second. Then something outside the parks:
It depends on what you are into:
sports: golf, mini-golf, fishing, RPDE, marathon, bowling, ESPN Club, WWS/ESPN Zone, etc
shopping: DTD, area outlet malls, large scale malls
drinking: Jellyrolls, Raglan Road, monorail tour of resorts, walking tour of resorts in the Boardwalk area
movies: DTD 24 screen theater some with meals served by waiters
night clubbing: Jellyrolls, Atlantic Dance, Raglan Road, Citywalk
resort tours: monorail tour of resorts, Boardwalk around the lake tour of resorts, AKL/AKV
offsite: Gaylord Palms Resort, Citywalk, swamp tour