My DD collect pins - they are small (generally quarter size) and pretty easy to find, especially in Europe. She has a special fabric folder that the pins pin into and she can add - on to.
My son collects patches - more expensive though! He also has a book on planes that he has the pilot sign for each of his flights. Some comments are funny, some are short and to the point, but he loves re-reading them and reminiscing about the flight and trip.
FWIW - DD is 10 and DS is 7.