It depends on the production, but Polonius's death and Hamlet hiding the body has a piquant element of Black Comedy.
"Good night, mother."
Exeunt severally; Hamlet dragging in Polonius
Followed by Hamlet saying that if they don't find Polonius's body, by next month, going up the stairs to the lobby they'll "nose him".
Polonius' death. Upon being slain by Hamlet (read: stabbed through a window curtain), Polonius feels the need to exclaim "I am slain." Something that this made this troper reconsider the play's genre. It just feels like a comedy after that (in most adaptations, if not proven in the text) over-fed, noble man points out his own demise as it's happening.
To be fair, he would not have been visible to the audience.
Polonius going on... and on... and on... and saying "brevity is the soul of wit"... and on...
The actor playing Hamlet will often add to the performance by emphasizing the first syllable.
"Do you think I mean CUNT...rrrrrrry matters" - David Tennant as Hamlet
Plus, it's widely agreed that this was entirely intentional on Shakespeare's part. The man had an absolutely filthy mind.
Whenever Hamlet is messing with people, especially Rosencrantz and Guildenstern
When he calls them sponges
Being roundabout and unhelpful as to the location of Polonius' body - "The body is with the king, but the king is not with the body. The king is a thing-" *cut off by Guildenstern*
The First Quarto note Once considered to be the 'bad version' of Hamlet, but now seen as snappier and more logical in the scene order, as it's most likely a memorial reconstruction by one of the actors in Shakespeare's company. The accuracy of the text noticeably jumps up whenever Voltimand is onstage. has a variation on the graveyard scene. Only a few lines after "Alas, Poor Yorick", Hamlet jumps into Ophelia's grave after Leartes, who attacks him:
Hamlet: I prithee take thy hand from off my throat,
For there is something in me dangerous Which let thy wisdom fear. Hold off thy hand! I loved Ophelia as dear As twenty brothers could. Show me what thou Wilt do for her. Wilt fight? Wilt fast? Wilt pray? Wilt drink up vessels? Eat a crocodile?
The entire graveyard scene given that it is Hamlet and Leartes having what can only be described as an Emo-off.
Depending on how it's played, Polonius' escalating frustration with Hamlet's "antic disposition". ("You seek the Lord Hamlet? Well there he is!")
Hamlet is just generally underrated where humour is concerned. Hamlet spends much of the first half of the play and some of the second half gleefully and hilariously messing with Polonius, Rosencrantz and Guildenstern, and Claudius.
The RSC's version of Hamlet with David Tennant had the point where Hamlet whistles along to the theatre troupe's triumphant fanfare, including a vibrato high note at the end.
Also the face he makes for the final 'than death' when Polonius takes his leave.
Fortinbras and his army arriving at the castle after Hamlet, Laertes, Claudius, and Gertrude died. As he sees all the dead bodies on the floor, he says: "Where is this sight?", or in plain English, "WTF happened here?"
The main gravedigger is a great source of humor, since he is an absolute Deadpan Snarker.