Act 2 is the act in which the most intense rising action occurs, leading up to the climax. The rising action of a plot is the moment when the action in the story begins to build. It is always the point when the story begins to become exciting and suspenseful and leads up to the climax, which is the most intense moment of the story. Since most of the rising action occurs in Act 2, we can say that Act 2 is the scene in which the characters have the most fun out of Acts 1 and 2.
In Act 2, we are introduced to the fairies for the first time who cause the most mischief in the play. In fact, Oberon, and especially Puck, are responsible for muddling up the lovers so that now both men are in love with Helena instead of Hermia. In Act 2, Oberon tells Puck to bring him the flower that has been enchanted by Cupid's arrow so that Oberon can use it to trick his wife into giving him the Indian boy he is jealous of. In addition, Oberon sees how cruelly Demetrius is treating Helena and also tells Puck to use the flower to enchant Demetrius. Act 2 is also the moment in which Puck mistakes Lysander for Demetrius, not knowing that there are actually two Athenian couples in the woods that night. Puck's mistake leads Lysander to fall in love with Helena instead of his true love Hermia, which leads to the intense, emotional climax of the story. Not knowing that the couple he was looking at was not the one Oberon was speaking of, Puck interprets the fact that they are sleeping so far apart as a sign of Lysander's hatred for Hermia, as we see in Puck's lines:
Pretty soul! she durst not lie
Near this lack-love, this kill-courtesy.
Churl, upon thy eyes I throw
All the power this charm doth owe. (II.ii.76-79)
Hence, we see that Oberon has a great deal of fun enchanting his wife in this act while Puck has a great deal of fun using the flower to remedy a situation he perceives as wrong, ultimately leading to the climax.