We loved seeing the 1994 version of the iPad so much that we decided to look into who had an iPad-like tablet first.
A number of "Star Trek: The Next Generation" episodes included tablets -- why are we not surprised -- but the earliest one had to be in an episode from 1989. In this clip, a boy watches a video on a flat, thin gray tablet with weird black buttons/strips at the bottom [ see above]. Note that he doesn't touch the screen.
However, some could argue the first iPad debuted in 1968 in "2001: A Space Odyssey." The movie shows a familiar scene of watching a video while eating.
This tablet also does not appear to have a touch screen, but it was close enough to an iPad for Samsung to claim in its legal battle with Apple that, essentially, director Stanley Kubrick invented it.
Compare the "TNG" and "2001" screen shots: