Goldorak has given some good advice. Here's a slightly different view.

First, understand that especially now you have no idea how messed up things will be at the airport so you your eight hours may be seriously cut.

The Métro does not stop at the airport. You can take the commuter train known as the RER, but as the US Embassy notes:



There is a bus called RoissyBus that operates from the airport to the Place de l'Opéra. From that point, the Métro can take you anywhere. Since you mentioned Uber, on my last visit, a year ago, I found myself taking G7 more frequently. They, too have an app.

In terms of what to do in Paris, I personally would vote for a great lunch and just walking around the streets. Or, select one very famous site, e.g. Arc de Triomphe or Eiffel Tower and see that.