Bar Brawl: Hawkgirl loves these, as seen with her idea of a "celebration."
Breather Episode: this episode is the only oneshot in a series of two- and three-parters, and would be immediately followed by the Season Finale.
Cats Are Mean: Streaky hisses at J'onn on sight. Averted in the end, though.
Children Are Innocent: The Ultra-Humanite is contemptuous of humanity in general, but thinks that children have not yet been corrupted into their elders' shallowness. This motivates him to fix the orphans' toy (and modify it to tell the story of The Nutcracker in place of its usual litany of hip-hop music and rude noises).
Pet the Dog: Flash's words move the Ultra-Humanite to fix the toy and give it some welcomed improvements.
Present Peeking: Superman/Clark Kent's parents mention how young Clark used to peek at his presents with his X-ray vision. Later that night, (adult) Clark sneaks into the room with the tree—and finds his presents are all wrapped in X-ray-blocking lead foil.
Seen It All: The Kents come across as a bit jaded when they say that they're not strangers to aliens in the house.