(while writing code, because no book will teach you to do :))
Advent of Code is also a very good source for exercises, especially if you want to learn the standard library. There's a lot of opportunity to use the very useful methods on Iterable/Enumerable/Indexable/Array.
I have a problem with AoC even with the language I've been using for ~20 years...not very good at human descriptions of problems with tons red herrings
I tend to use Crystal pretty often to implement math routines that I find interesting (e.g. I've recently been interested in Lucas Sequences for estimating roots), so that could also be another way to practice, if you find that interesting.