Any advice on traveling to a Bruno Mars concert at Stade de France? Assuming metro would be the best, but what about Uber/taxi? but any suggestions? Are there cafes near by to go early? How insane will it be after the show, returning to central Paris ?
Take the metro. It is an easy trip.
There are restaurants nearby as well. Use Google maps to see what you can find in the environs.
The RER D goes to Stade de France. The RER is a huge commuter train so it would not be as crowded as a metro -- and if it is, you just wait for the next train. We often go by metro to the Philharmonie or to the Opera Bastille and often wait for the second or third train 'home'