Depends a little on when you're going. If it's during a busier time of year when the parks are all open late, spend a little time getting settled in your room, maybe have lunch there, and go to a park mid PM when the crowds are thinning out. You'll still have lots of time to see things. If it's at a slower time of year, most parks will close early, but Epcot is always open til at least 9, so it would be a good bet for a partial day.
I've used these boards to plan my last two trips (both land/sea cruises) and found it invaluable for both the WDW and the cruise portions. You hardly have to ask a question - just read the site and discussion boards and you'll find tons of information. My other indispensable source was the Unofficial Guide to Disney World guidebook. Between these boards and that book, you'll find everything you could want to know. And don't be afraid to ask questions here - everyone is always happy to help.
Enjoy your vacation planning.
