`xeyes` will render two eyeballs that follow your mouse cursor. Though I've used it more for entertainment, testing X11 forwarding, and testing configs with `startx`.
`xev` outputs useful information about X11 events, including keypresses and mouse movements. It's useful for discovering the names/codes of keys for keybindings. And running it can even help you recover if some program has captured X11 input and hasn't released it.