During Holiday Time at Disney California Adventure, Mickey, Chip, Dale, Goofy and Pluto all appear on Buena Vista Street in their Christmas versions of their regular BVS outfits. The Citizen of Buena Vista Street also have some small modifications to their costumes (but only minor).
Over at Paradise Pier you can meet Donald and Duffy (separately) in special holiday costumes.
In Cars Land Tow Mater dons a Christmas hat, and Santa meets guests at Elias and Co on Buena Vista Street.
Phineas and Ferb's show and the World of Color have slight modifications for the holidays.
You can find the talented Community Bell Ringers playing some well known Christmas tunes on Buena Vista Street, and also join them and the characters for a special Holiday Tree Lighting every evening at the Carthay Circle.
Don't forget that much of the park is decorated for Christmas, and you can find special holiday treats and gifts throughout the park's eateries and stores.