Code Reading
Rapid, effective code reading is a useful skill for all programmers. For example, before you can refactor, you need to read the code and understand enough of the current structure of it that you have a good idea of what you can change safely.
I found out about this concept of teaching Code Reading from Felienne Hermans and this is a useful blog post she wrote about it. I attended a session she ran and adapted some of her exercises for the learning hour format.