MK for the first FULL day, as said a few times. Jenn Lynn says it well. Something magical about entering MK in the AM.
I am assuming you have AP's if there for 8 nights. If so, no limits on what parks, how short or long. You might take a bus to the TTC, a monorail loop around Seven Seas Lagoon, just to get a 'neat fuzzy feel' of WDW, the monorail, the Poly, the GF, the magic of MK that awaits you in the AM of the next day, a cruise through the Grand Canyon Concourse of the Contemporary. (Yikes, I am getting homesick

). After that tour and your mind all bedazzled, switch monorails at the TTC and head to Epcot on the Epcot monorail. See if you can get to ride with the pilot and get a pilot's certificate. Now, at Epcot, enjoy the late afternoon, a bit cooler, and on into the evening, Illuminations, a slow stroll back to the buses and ASMu. Up, bright and early for the magic of the Magic Kingdom.
(UGH, I need to pack!!!!!!!!!!

) Enjoy, Mermaid, you will forever remember your first visit to WDW!!!!!