How about Pikachu's reaction when Ash and the others are giving him "sinister looks" before dressing him up?
And when Pikachu fights his first battle, his Thunderbolt attack destroys the outfit — which, BTW, is a rental. Does the palace tailor stick Ash with the bill for that?
During the opening credits, Mew transforms into the queen's Mime Jr. and watches the tournament alongside them. The maid does a WildDouble Take when she realizes she's seeing two Mime Jr.
Brock gushing over Kidd, with the queen's Mime Jr. imitating his every move.
There's something entertaining about watching Ash and Lucario wrestle in a river.
Max trying to get Lucario to enjoy chocolate. When Max turns his back (or pretends to), Lucario smiles only to frown again when Max looks back.
Team Rocket seeing the Tree of Beginning being restored.