My great, great Aunt Reta had a friend named Lilian who liked to collect stamps. She'd send for them in the mail, and they'd arrive in little wax paper envelopes.
She also corresponded with friends and family that lived overseas and in various parts of Canada and the United States, and kept the stamps from those letters, too.
And whether she meant to or not, Lily collected postmarks. I find these to be just as interesting as the stamps, and each one very telling of a particular historical moment.
I need to write more good old-fashioned letters, don't you think?