In DL, Town Square is a good place to find characters. Ariel has a grotto between the Buzz Lightyear ride and the Matterhorn where she greats guests. Aladdin and Jasmine (and often the Genie) can be found in Adventureland (often Timon, Rafiki, Jane and Terk are around too). Buzz can be found at Club Buzz. Woody and Jessie are in Frontierland, often around the entrance to Pioneer Mercantile. Pocahontas is sometimes seen in Frontierland, along with Goofy or Pluto in western wear. Princesses are often near the castle, either near the Plaza Gardens or Snow White's grotto. Pooh and friends are in Critter Country. In Toon Town, you can meet Mickey in his Movie Barn, Minnie in front of her house, and other characters in various other places. There are some suprising spots to find them. Last night, I was waiting in the wheelchair line for Space and Mickey came out and spent some quality time over there (I've seen many others there--Goofy, Pluto, Minnie, Chip & Dale). Once, in the Starcade, I saw Dale and Geppeto playing air hockey. I've seen characters out just inside the main entrance gates and over by Small World also.
In DCA, fab 5 characters are usually found in the Sunshine Plaza area. Various movie characters are along Hollywood Blvd. Chip & Dale are near GRR.