In one Sunday strip, Garfield tells Jon he wants lasagna for dinner, but Jon, being unable to understand Garfield's thoughts (in this strip, anyway) suggests things like steamed rice. Garfield, furious, walks away and comes back with ingredients, throwing them in Jon's face. Jon FINALLY decides on lasagna.
In one strip, Jon deliberately denies Garfield kitty treats because "life isn't fair." Garfield's response? Yank one of Jon's neck hairs.
Jon has one in an early strip where gets Garfield to take his bath. He tries to wrestle Garfield into the bath, and when he fakes defeat, Garfield jumps off the table and into a second tub of water.
From one series of 1999 daily strips: Garfield throws Nermal out the door three times, then flushes him.
In one comic Jon, Liz, Garfield, and Odie are at the beach, when a guy named Rolf kicks sand in Jon's face in an attempt to steal Liz away from Jon, only for Liz to tell him to get lost with Garfield also siding with Jon. A similar moment happens in the 80s where Jon tries to flirt with a girl at the beach, only to get rudely shot down by her. After Jon leaves, Garfield throws sand at the girl.
Another moment between Garfield and Nermal: two days after Garfield's 40th-birthday, Nermal (instead of insulting/making fun of Garfield's age and/or appearance) notes that Garfield actually looks pretty good, to which Garfield says, "Aging well is the best revenge."
In this strip, Garfield sees a cat knocking Odie on the head. What does Garfield do? Punch the cat into the sky!
Garfield: "Nobody beats up on Odie but me."
The very first TV special, Here Comes Garfield, Garfield working with the other cats at the pound to save Odie from being euthanized.
In the prime-time TV special Garfield In the Rough, an escaped panther threatens the life of Jon and Odie. Garfield, who ran up a tree in fear, has a sudden Papa Wolf moment, dives from the tree onto the panther's back, clawing away at it yowling and snarling like, well, an enraged housecat. note This is actually one of the very few times Garfield makes real cat sounds. He doesn't actually defeat the panther, though he does buy park rangers some time to tranquilize it without anyone getting hurt. Doubles as a Heartwarming Moment that Garfield would actually risk his life for Jon and Odie, especially since a few of the jokes in the comics and animation are about Jon specifically asking Garfield if he would do just that, and getting turned down.
Jim Davis's artwork deserves a mention, particularly in the special's tie-in book; the panther looks positively frightening in some of the panels, and the panel showing Garfield leaping from the tree, claws and fangs bared, gives him an absolutely badass moment.
In the Halloween Special, as Garfield and Odie jump into the lake to escape the evil pirate ghosts, Garfield almost immediately sinks under and is swept away by the current, as he can't swim. Odie immediately dives underwater to find him and pulls him to safety. This doubles as a Heartwarming Moment, because he could have chosen to just save himself, but he cares too much about Garfield. To add to the heartwarming, Garfield, who was originally going to take all of Odie's trick or treat candy, states that he can keep it and have half of his own. Garfield gave up some of his food as a thank you.
Speaking of the animals, the animals he met at the pound and the city rally beside Garfield to take down Happy Chapman. He then rounds up the dogs, cats (off-screen), and rats to make the attack, and they dogpile Chapman in a hyenas-attacking-Scar style.
Garfield flat-out hissing at Happy Chapman "Get your hands off of my friend [Odie]!" and leaping at him. It's a mix of this and a heartwarming moment, having finally befriended (somewhat) the dog he disliked throughout the movie.
Jon does another knock-out punch in the sequel as well, and that time seemed even more awesome because the Big Bad Lord Dargis had Liz at gunpoint up until a few seconds prior. Also note that in both movies, Jon simply needs to deck the Big Badonce to render him unconscious.