1. Copy - Paste text without formatting

Pressing Command-C copies text to the clipboard, and then pressing Command-V pastes it along with any formatting.
However, if you want to paste the copied text elsewhere as plain text, use the key combination Command-Option-Shift-V and any formatting will be removed.

2. Quickly convert an image in Finder

You can convert one or more images directly from the Finder using Quick Action.

Quick Actions help you perform tasks, like creating a PDF or rotating an image, directly from the Finder or desktop, without opening an application. With Convert Image, you can quickly convert an image file from one format to another, as well as resize it.

3. Copy - Paste image editing

If you have multiple photos that you want to edit in the same way, or if you've made changes to one photo that you want to replicate in another, you can use the copy and paste editing tools in the Photos app.
To use this option, first open an image, click “Edit,” and make any changes you need. When you’re done, choose “Image” > “Copy Changes” from the menu bar, then choose the image (or images) you want to paste them to. Finally, click Image and Paste Changes from the menu bar.

4. View Calendar Events as a list

Click the Search field in the upper-right corner of the Calendar window and type two double quotes ("") to create a list of all your upcoming events.
This tip makes it easy to copy multiple events and paste them into other applications in chronological order.