Raspberry Pi Model 3 is a computer the size of a deck of playing cards.
A great route into learning about aspects of computing, it can be connected to a TV or PC monitor (using a HDMI adaptor). The Raspberry Pi foundation provide several different operating systems to try out which are all available online.
You can learn about the different inputs and outputs of a computer as well as attaching various different outputs to the GPIO pins and program what they do, for example, a traffic light. Sold with or without an SD card, which acts as the computer's memory.