I don't add a lot of customization to my Mac, but these I have found to be very useful:
Doll to show messaging apps' icon and badge in the menu bar. (I turn off all notification sounds and banners.)
cd to app Finder toolbar extension that opens the current directory in the Terminal.
The recent discussion Taking command of the Context Menu in macOS on Hacker News has good advice on configuring the Finder's context menu with the ContextMenu, Automator, and other tools.