Chakotay needing to convince past Janeway to trust him and follow him through various events of past episodes was pretty interesting.
The DS9 episodes were just fun and I feel like there's not many that go back to that time period like Little Green Men did. Most time travel episodes either go further back (1800's, WW2 itself, etc) or are set in the 'present day' (Voyager's/Enterprise's/Picard's).