Appearances in Christmas specials
The Adventures of Jimmy Neutron, Boy Genius
In the series' Christmas episode, "Holly Jolly Jimmy", Cindy takes part in singing "Basking in the Warmth", and then scolds Jimmy when his claim that Santa Claus isn't real hurts Carl's feelings. Later, while peeking at their Christmas gifts, Cindy and her best friend, Libby, decides to take an embarrassing video of Jimmy to get back at him for upsetting Carl. They end up being trapped in Jimmy's Hypercube and taken to the North Pole. After using Libby's portable CD player to escape the Hypercube, they manage to record footage of Jimmy's DNA tracker firing a beam that scrambles Santa's atoms. Cindy decides to use this to humiliate Jimmy and then formulate a plan to save Christmas (which she admits she has not fully figured out yet), but Santa ends up giving lumps of coal to her and Libby.
Other Christmas productions
A year prior to "Holly Jolly Jimmy"'s premiere broadcast, Cindy also appeared alongside her companions in the Merry Nickmas short "Patrick the Snowman". In that segment, she, Jimmy, Carl, Sheen, and Libby build a snowman that looks like Patrick Star, with Cindy commenting that it looks hideous. After Patrick is brought to life by Jimmy's Spoofinator 4000, one of the ways he annoys the kids is by sneezing ice cubes in Cindy's face. She suggests that they just melt Patrick somehow, but the kids instead send him 70 years into the future, where he has started annoying the now-elderly kids, Cindy chastises Jimmy for his idea.