I can't think of any danger in this city. It is really calm and safe. The only danger is that you do just come with time enough and leave after visiting the cathedral. You would miss then a lot of interesting things.
It is completely dangerous and nonsense to visit Reims, go to a local pub/bar and ask for a "Leffe" - the most famous Belgian beer.
The waiter will shoot you down, but the drink will be for free.
using the restrooms in Mc Donald's, they are still the typically french and rather complicated to use ones, where you can just stand there and....
they are not very clean either