OS X is great for having little hidden gems throughout. It's no surprise when I recently discovered this one... an easy way to display a welcome message on the login screen.
Whether using this for self promotion or security, it's definitely a handy gem. To enable the feature, open System Preferences then click on the "Security & Privacy" button. From here, check the box titled "Show a message when the screen is locked" (if this is grayed out, you will need to click the yellow lock in the lower left-hand corner to unlock the settings).
After checking the box, now click on the "Set Lock Message..." button to enter your custom message. Feel free to use emoji characters in your message as well!
Now, when you turn your Mac on (or any time your screen is locked), users will see your custom message! Here's another example of what this could look like (this one is more simple and doesn't include emoji).