Mild amusement at it's best. Most experienced Linux users will know these, so this for the noobs. These are all command line based, so open a terminal
1) Steam train rushing across the terminal:
Instead of typing: ls to checkout the content of a directory, type: sl
2) Use your mouse zap those pesky penguins invading your screen:
Type: xpenguins -n 35 -s
If it comes up command unknown then: sudo apt-get install xpenguins
3) Cat chasing mouse. Get a cat to chase your mouse round the screen.
Type: oneko
If it comes up command unknown then: sudo apt-get install oneko