Doc on the Clock Tower?
In the opening credits when the camera is panning across the clocks, one of them is a model of the Hill Valley clock tower complete with a Christopher Lloyd action figure dangling from the minute hand.
Note: Others say this is an old novelty clock from the 1920's depicting a scene from the Harold Lloyd film "Safety Last." The scene, in which Lloyd dangles from the clock high above the street, is probably the most famous image of the silent comedian.
Sherman & Peabody
In the scene where Marty enters 1955 and crashes into the barn, the owner of the farmhouse is Peabody. He refers to his son as Sherman. Sherman and Peabody were two time-travelling characters that had their own cartoon series and often appeared on the Rocky and Bullwinkle show.