The assault on the castle, which is fully the princesses' time to shine. Of particular note is the beginning. Snow White hops out and starts singing happily and luring out a bunch of woodland creatures... then her happy trilling turns into the Immigrant Song, making the animals attack the guards.
Everyone showing up to back up Shrek on stage. Including Dragon & her kids flying in formation and the 3 pigs in martial art poses. Yes, Charming ruins it by calling in the villains, but still...
Artie's speech at the end, convincing the other villains that, yes they do have a choice. Fits well with the series idea of "There is always another ending".
When Artie gets picked on by the students, Donkey of all people (having been stuffed into a locker and having a "I sucketh" note planted on his butt) snaps at them "Ya'll should be ashamed of yourselves!" Defied quickly when Lancelot says "That wasn't me." Still, a funny talking donkey standing up against the bullies is kinda cool.