Foster's Home for Imaginary Friends (2004)

S6E9 The Bloo Superdude and the Great Creator of Everything's Awesome Ceremony of Fun That He's Not Invited To

Bloo, feeling feverish and unwell, finds himself at Mac's birthday bash and begins to fantasize about his life as a superhero. As he drifts in and out of reality, he imagines epic adventures filled with excitement and heroism, all while surrounded by the festivities of the party he didn't plan to attend.