Alie and Jaha talk in the City of Light in episode #302 "Wanheda, Part 2". Alie tells Jaha that Raven knows how to destroy her in episode #311 "Nevermore". Jaha and Alie use Monty's mother to slow Raven's attack in episode #314 "Red Sky at Morning".
The Trickster poisons people at the Mayor's press conference that Iris is attending in episode #117 "Tricksters".
Cassandra trains with some soldiers before being kidnapped by Snart and Rory in episode #113 "Leviathan".
Source: VanFlarrowFan — Twitter
Kiera arrives in the future at the feet of Kagame and is then greeted by future-Alec in episode #406 "Final Hour".
Cyborg Superman breaks into the lobby of L Corp to steal the isotope needed to use the Medusa virus but is thwarted by Kara in episode #208 "Medusa".
Source: What's Filming