Although it has a reputation for having a steep initial learning curve, in fact Linux / UNIX is great fun to learn and use, and obviously much more is possible with its command line due to the many decades of constant evolution and improvement that have been put into it by the thousands of active developers in the open source community.
This makes it very rewarding to use.
Here are some commands that are purely about fun and serve little to no purpose in the serious workplace. They can each be installed using apt-get install or yum install depending on whether you’re using a debian or redhat based distribution.
moo – displays a cow
sl – lists files in a directory like ls but if you get the syntax wrong, a train will drive across your screen
figlet – useful to turn text into big chunky letters
cowsay (and xcowsay) -displays a cow that says whatever you tell it to say as text (or graphic)
oneko – displays a little ASCII cat that chases your mouse around the screen
rev – reverses text
fortune – displays random quotes/nonsense. Can be used in conjunction with cowsay e.g. fortune -s | cowsay