The song "Only Thinking Of Him", which consists of Alonso Quijano's niece and housekeeper talking to the priest about how worried they are for him, when it's blatantly obvious that they're primarily acting on selfish reasons. The priest responds with delightfulsarcasm via exaggeration.
The entire play is performed by a cell full of criminals. Watching grubby scraggly miscreants acting like ogres, gypsies, and priests is hysterical.
Within the show itself, "Knight of the Woeful Countenance" and "I Like Him" are hysterical.