Terminator 2 Thumbs Up
This easter egg can be seen in Episode 4.
When Ryuko, Mako and Maiko are running across the pot full of lava, we see there are many bones inside floating on the surface. One of those bones is none other than that of The Terminator, or more specifically, his hand at the end of Terminator 2.
The easter egg is made more apparent once you pause and zoom in as you’re then able to see it’s not just a floating hand, but that the hand is making a thumbs up, exactly the same as in Terminator 2.