I'm only counting "walking through the park" experiences, because celebrities are there all the time doing scheduled shows and stuff.
John Goodman in the Magic Kingdom, all members of N'Sync together walking around the World Showcase (then Joey Fatone and his family at Test Track on a different trip), Howie Mandel on Main Street, Hayley (sp) Mills on Mr. Toad's, Jay Kay in MGM, Brandy outside Fantasmic... actually, I can't think of any right now. All I've got stuck in my head is the MSEP Pete's Dragon for some reason. No, I'm not kidding. I'm trying to think of people and I flash on Dolly Parton for a second then I see the smoke coming out of the Dragon's nostrils.
I did go up and talk to Brandy, though. She was SUPER nice, and no one else seemed to recognize/notice her so I kept my voice low. And Howie was talking to everyone so that was kind of unavoidable (not that it was a bad thing - he's a funny guy, he was just all over the place).