In the first episode, Discord is out flying around Ponyville. Pinkie sees him, and rather than be freaked out, she starts talking to him. Justified as she thought she was dreaming, but it's still hilarious.
Derpy singing. That is all.
After the dodgeball challenge, Gilda gets so mad she tears apart an entire gymnasium from the inside out.
And later, after being voted off, Discord has to pull her away in a strait jacket.
In the talent show challenge, Princess Celestia and Luna have a "yo momma" insult contest. The concept alone is hilarious, but the context makes it even better.
Speaking of the "Yo momma" contest, Celestia dragging Luna offstage to give her a spanking for actually bringing up their mother during the contest is funny as well, it's even funnier when you realize that because of Celestia's "condition", Luna is bigger than she is.
Spike spying on Rarity can come across as pretty funny, especially when he tries to pass it off as him watching a rare bird.
During the food-gathering challenge, anytime some of the explosive food goes off usually ends with funny results.
Three times at the elimination ceremony, Discord says that whoever is voted out must leave the island immediately and may not come back, ever. Pinkie Pie proceeds to cut in with "FOREVER!", which actually creeps Discord out. Except the third time, in which he just goes and bangs his head against a tree.
A few of Rainbow Dash's schemes for putting her opponents to sleep in the challenge for staying awake are this. For example, walking up to Derpy, very conspicuously picking up a rock, right in front of her, asking if it's true that things hitting her lightly in the head knock her out, then chucking the rock at her forehead. The entire time, Derpy doesn't realize it's sabotage.