Adobe Photoshop Only
Apply Image is easy to master once you know how.
The best way to retouch skin using Frequency Separation with Apply Image.
There is a still a place for Calculations in Photoshop, especially when it comes to creating masks.
Done for fun only
The best books, in my opinion, for learning Photoshop.
Color Efex Pro is no longer being supported by Google, or by Adobe. Use it before you lose it.
Adding a sun flare, or starburst effect using a custom brush, and Layer Styles.