Postage and envelopes (called covers by collectors) have always been hugely interesting to me. I appreciate stamps for their cultural and historical significance, and also, in many cases for the incredible feat of turning such a small canvas into something beautiful.
It used to be — it may still be the case — that if you went to a stamp show and spent time going through the boxes of covers you could find envelopes that had gone through the mails not only with interesting postage stamps and cancellations and handwriting, but also with letters and enclosures still intact. I guess people sometimes just sell everything in an estate, including correspondence, and as a result I have come across (for example) a letter written to an Aunt Caroline in Iowa in 1949 that included photos and drawings. Whole stories — whole novels — can jump out at you from something as simple as an envelope. Like this one, which comes from The British Postal Museum & Archives weblog. Taken together, the stamps and handwritten notations are quite somber. It's the "addressee reported prisoner of war" that strikes home.
There's no lack of inspiration out there when it comes to storytelling.