Airport codes are one of my milder nerdy interests. I like learning them, and knowing at a thought, say, what the code is for Hong Kong (HKG) or Delhi (DEL).
Link: World Airport Codes.
Tip: Airport codes also come in handy when you’re booking travel — less
typing than the city name, plus when you get to the airport you can at a glance verify that
your bags are getting tagged for the right cities.