I did one day at WDW a little over a year ago. We started the day in Epcot and then park hopped to Magic Kingdom. We definitely didn't see everything in either park, but for one day it was cool to get a little of each. What are your preferences when in the parks? Do you prefer shows and parades, thrill rides, or gentle rides? Are you more interested in characters, animals, or eating good meals? I would personally start the day at Epcot or Hollywood Studios (I love the thrill rides at these parks) and stay at the first park through lunch. Last year, we chose Epcot because we wanted to have lunch in the World Showcase and see some of the countries. Then, I would park hop to Magic Kingdom to finish out the day.
My planning last year was a little too fast paced for my group, so if I was doing it again I wouldn't plan for more than 2 attractions in an hour, leave an hour each for lunch, dinner, and snacking in case you feel like sitting for a while, and give yourself an hour to hop parks. You'll probably do more than this, but you won't feel like you're rushing around too much!