I really like the Kotlin Koans:
https://kotlinlang.org/docs/koans.html I prefer the IntelliJ version to the online version since you get immediate warnings if you make syntax mistakes.
Also notice that the Koans are part of a whole section on Learning Materials 😉
I leaned Kotlin by doing the Kotlin Koans and by reading Kotlin in Action and I loved both.
There's a second edition of Kotlin in Action on the horizon, but I'll probably take another year to come out. But if you preorder it now, you'll get the first edition as an ebook:
https://www.manning.com/books/kotlin-in-action-second-edition