My boy is 11 now. We've used Anki with him every day since he was about 5.
Examples of things he's memorized, for fun:
* every country, recognized by unlabeled shape on the map
* spelling
* many body parts, names of bones and organs, from illustration
* chemical elements, by symbol (Na, Fe, Zn, etc)
* unix filesystem commands
* numeracy references (km from here to Japan, earth to sun, meters from home to school)
* recognizing/naming photos of places we've been since he was born
* recognizing musical instruments or musical pieces by listening
* notes on a piano
* religious facts (when Judaism began & who started it, when Muslims pray & towards what, where Jesus was born & died, etc)
* names of characters in books he's read
* wise aphorisms
He enjoys it, and dances around while answering, proud of himself. Sometimes when learning something new on YouTube, on his own, he'll say, "Dad can we add this to Anki? I want to remember this."