Unsubscribe or mark as SPAM.

Select messages and mark as Spam.

Snooze an email:

Schedule and email to send:  Sometimes you want to compose an email but have it send at a later date.  Click the arrow next to the Send button to Schedule send.

Create filters and labels for your mail.

Undo Send:  How many times have you sent something and as soon as you sent send, you want to retrack and edit or add something?  

Right after you send a message, you can retract it:

In the bottom left, you'll see "Message sent" and the option to "Undo" or "View message".

Click Undo to cancel sending the message.

Check your Settings to increase the time limit to Undo Send.  30 seconds is the maximum.

Clean up old messages:  If you haven't deleted old messages in a while, you can do so with one fell swoop!

type (in the search)  before: yyyy/mm/dd

and Select All to  Delete or Archive

Signature Line:  Add a signature line or quote.

Vacation Reponder:  Message for if you will be out for a few days.  Can also be found under Settings/General.