The 1994 Knight-Ridder video I attached at the bottom of this post is a fantastic reminder that the tablet predates the iPad and Android tablet by many decades. During the "hypermedia" era of the late 1980's, I can recall taking a "tech of the future" class where my professor discussed in similar detail what a tablet might look like in the future. He described a day where students would be sitting under trees reading not from paper books but utilizing exactly what we know today as the digital tablet.
Believe it or not though, the origins of the tablet computer date back to the 19th century.