Flynn Rider and Rapunzel make an appearance at Elsa's coronation in Frozen.
Mike Wazowski from Monsters, Inc. appears as a small figure in Frozen
Hans from Frozen shows up in a wanted poster during Big Hero 6.
A little Mickey Mouse doll also appears in Frozen.
Disney comments on Kristoff's assertion that all men eat their own boogers at the end of the credits of "Frozen".