when you get there, the park guide maps show Character meet & greet areas...
truthfully, they're like all over...
MK: Main Street area (Tony's Town Square, City Hall, etc.)
Tomorrowland, main walkway from hub
Frontierland, in the Golden Horseshoe theater
Fantasyland, Ariel's grotto
etc.
MGM: around the sorcerer's hat
down the path to the left of the hat (Playhouse Disney Characters)
outside Muppets (Al's Toy Barn)
Animal Kingdom: conservation station
off the path on the way to Dinoland (Pooh & friends usually there)
Epcot: there's a nice organized set up near the innoventions (right side, on way to the Land)...
When we were there in Feb. Chip & Dale were fawning all over a little infant in a carrier, quite the site...
and of cource character meals