For me it would be code reading and reviews.
I have been realising that reading code on many different projects is a great way to learn and keep up with how others solve problems. Reading the code of the libraries you use also makes you use them better and understand why things work in a certain way. It has been rewarding and I want to do more of it.
And as I read more code in many different, it has also been helpful in improving my code review skills.