Airport bus
If the airport is not your starting point or final destination you might want to catch a lift. Many airports, especially those served by Ryanair - farm houses with a runway in the middle of nowhere - offer a shuttle service from the airport to the city centre. Most of the bus drivers have no problems taking your bike on the bus. It makes sense to keep the luggage tag on the bike and show that it belongs to your luggage. I have never been sent away myself, sample airports being Istanbul SAW, Helsinki HEL, Stockholm Skavsta NYO. Friends of the railway will be happy to hear that there is a metro line to Istanbul Airport IST where you are allowed to take your bicycle, same for Edinburgh EDI. There is a metro line to Athens ATH, too, but I was very close to being thrown out with my bike. On the way to the airport you can always argue that you have to catch your flight.