custom commands with IFTTT:
https://www.cnet.com/videos/how-to-create-custom-alexa-commands-for-amazon-echo-using-ifttt/

Change language:  alexa.amazon.com  settings > select device > language
                  Or in app

Bluetooth (incl. for speaker)
pair bluetooth
connect to my phone
disconnect from my phone

Settings/ Alexa Devices > NAME / Sounds and Notifications > Start/End of Request

What's the definition of X

Music
play music by X
play [radio station] on tunein
what's playing?
next, restart, add this song

Audiobooks
read X / resume my book
read me my Kindle book

Timers/alarms
set an alarm to X at Y am/pm
set a X timer for X minutes

News.  Use app Flash Briefing > Get more FB content
What's new  (gets Flash Briefing)

Skills:
enable/disable X
open X  (automatically enables if doesn't require an account)

Search
Wikipedia X
  tell me more
How ...