As seen here, the Raspberry Pi is “rightly praised for its diminutive nature,” but the reality of things is that you’ll also need a keyboard, mouse, monitor, and USB power supply to get things running. Raspberry Pi boards can, however, run “headless,” allowing them to control other devices without requiring a display at all times, and user interface provided by remote login. This presents some interesting options, including turning a Pi Zero, or Zero W into a literal key fob, that you can plug into a remote computer via USB, or connect to over WiFi.
More details are available in the MagPi article, but the basic trick is that with a few changes to the Pi’s configuration files, you can set it up to operate headless and enable USB networking. With USB networking enabled, you can then plug the Pi into another computer, and log in as if you were accessing a remote system. With the proper software running on the host computer, like VNC viewer, you can even access the Raspberry Pi’s desktop, though you’ll want to blow up the default headless resolution.
Taking things a few steps further, if you have WiFi access you can connect that way, you could hook up a USB power bank or LiPo for a fully self-contained unit. Throw in a 3D-printed case, and you’ve got something you can take (nearly) anywhere, for Raspberry Pi fun on the go!