I think you could make a case for Lord Capulet, at one point in the play, to provide comic relief, and at another point in the play (by a very surprising turn of character) to provide action motivated by revenge.
In Act I Scene v, he's a rather jolly party giver, kidding around with his guests and servants -- even cautioning Tybalt that Romeo (their family's enemy) should be endured at the party for the sake of the festive atmosphere. This is a man who seems to be harmless and sort of silly. A good-hearted man who loves his family.
In Act III, Scene V, we are surprised to meet a very different man. At first, when Capulet enters Juliet's room to receive her "gratitude" for her impending marriage to Paris, he seems, again, light and tender hearted, even making fun of Juliet's tears, presumably to laugh her out of her grief. ("How now, a conduit, girl? What, still in tears?/Evermore showering?")
Yet, once it is clear that she is, in fact, not only not thankful for his "gift" of Paris as a husband, but even refusing it, he turns on her. "Hang thee young baggage, disobedient wretch!" Even condemning her (his act of revenge) to "beg, starve, die in the streets." It is worth noting that this scene is often played with Capulet taking his revenge out on Juliet with physical violence. If not an actual violent act, then at least the threat of one -- "My fingers itch."
Romeo and Juliet is often cited as the first play in the English language in which each character speaks in a truly individual rhythm, separate and distinct from the other characters in the play. And there is certainly nothing generic about Lord Capulet.
I think almost any scene with Mercutio in it could be considered comic relief. Nearly all of his lines must be taken "tongue-in-cheek" and if you read too quickly, you may miss half the things he's making fun of. I would also cite Act 3 as a part to go back to - not just for the revenge element, but also perhaps for the beginning of the scene with Mercutio.
The nurse is also meant to provide comic relief. It highly depends on how she is portrayed (on stage or in film) as to whether she is actually funny - but everytime I've seen her cast as a large woman who acts like she needs Juliet as much as Juliet needs her, the role reversal is always comical. The nurse tends to affectionately pick on Juliet (slowly answers Juliet's most pressing Romeo questions, pretends she has no idea what Juliet needs/wants, but then gets highly IMPATIENT when she needs Juliet - lots of extremes with this character) which is silly in a loveable way. Mercutio also makes fun of the nurse - go figure.
For comic relief, you might look at the Act 4, Scene 5, the scene in which Peter self-importantly gives orders to the other servants. This scene contrasts with the high drama and tension of the previous scene when the nurse and Juliet's parents find Juliet supposedly dead on the morning she is to marry Paris. The revenge motive is quite prominent in Act 3 when Romeo kills Tybalt to avenge the death of his friend Mercutio.