I introduced Doodlebug and Ticklebug to Calvin and Hobbes this weekend. DB can read it and TB is having fun looking at the pictures when there is no one to read it to him.
Ticklebug likes the ones in which Calvin pretends he is a spaceman. Doodlebug likes the "gross" ones as she puts it. Basically, the ones involving toilets, bugs, worms, vomiting and general "potty" themes. It is cool to see their sense of humor and ability to "get" a joke develop.