I went back and watched some DS9 earlier. This time I viewed the two-parter called "Past Tense". This was pretty good(although not good enough to make my Top 25 in the Star Trek episodes LoC). In it, Sisko, Bashir and Jadzia are accidentally beamed into the early 21st century(2024 to be exact). It shows a bleak version of... OUR future but the events were in part responsible for making the Earth a better place for transients and the mentally ill.
Hey now that I think of it, DS9 had TONS of time travel episodes like the aforementioned Past Tense, but also Little Green Men, Trials and Tribble-ations, Time's Orphan, Wrongs Darker than Death or Night, and Children of time.