The Liquidpubs team has published an article on the history of the tablets that Apple has made over the past 30 years. I recommend reading it after the jump as there is no waste, but remember that many of these devices were only concepts.
Bashful, 1983: This is the first experimental tablet that was never marketed and it was a bad idea from Apple.
24 Hour Mac, 1984: this tablet is another concept that gave the opportunity to enter text and select different options to use the stylus.
Apple BookMac, 1985: one more concept that offered the possibility of having a version with telephone and fax.
P2 Portable, 1989- Futuristic in design for its time, the P2 is a touchscreen laptop concept.
Figaro, 1989: an exceptional tablet designed by Giugiaro that, however, did not quite convince Apple.
Hooop: 1989: tablet designed by Figaro for Apple.
Second Figaro 2, 1990: it is a concept of PDA made for Apple from which soon after the Newton began to be marketed.
WorkCase in Station, 1991- A concept developed by Apple that involved the use of a tactile trackpad that could be used as a PDA.
Newton, 1991: this is the first concept of the MessagePad Newton, the founder of the PDA.
Pen Mac, 1992: a whole Mac with CD-ROM that finally ended up being one more concept.
PenLite, 1992- A touchscreen computer based on the PowerBook Duo.
Apple Newton Bic, 1993: beautiful PDA that did not have the expected success despite having managed to sign a page in the history of computing.
Apple iPad, 2010: It is the tablet that Apple markets today and that according to Jobs, is the fruit of several years of work:
In the original article you have more information about each of these devices.