It used to be that you could root them and put a rom like CM or Nexus on them, but they've locked that down pretty well. You can still sideload the Google Play Store app though. Plex works on it and it has a microSD card slot so it works pretty well as a kids media player for long trips. I have one mounted on my fridge with Google Calendar showing on it.
https://smile.amazon.com/All-New-Amazon-Fire-7-Tablet/dp/B01GEW27DA