Barry fights against Killer Frost and Deathstorm in episode 2x13 "Welcome to Earth-2".
Source: What's Filming
Marcy reads to David in the park and then says she is his boyfriend in episode 2x10 "21C".
Diggle meets with his local contact while Oliver keeps watch in episode 3x03 "Corto Maltese".
Kara heads to a downtown park where an alien is shooting up the place with a laser eye patch in episode 2x17 "Distant Sun".
Olivia escorts Jessica Holt to a squad car and hands over her cell phone in episode 4x21 "Brave New World: Part 1".