> now I'm not even sure if I need a passport to cross the border to France for a day trip. You don't, in theory. However, within a certain zone from the border

> now I'm not even sure if I need a passport to cross the border to France for a day trip. You don't, in theory. However, within a certain zone from the border border agents have the right to ask you for identification. If you can't provide any they will take you along to determine your identity. So while it's usually not a requirement to pass a border within the Schengen zone I highly recommend that you have proper identification on you. Same goes for flying, at least before Covid. It was really rare that I even had to provide id within intra Schengen flights and if I was asked it was usually by the airline at the gate. The normal process was checking in automatically and then just badge yourself through the various checkpoints up and including the plane (depending on airport). Still, I would not recommend not to have a valid id or a passport when you fly. No matter if it's strictly required or not.