Postcards
I've been collecting postcards since I was a child. They're cheap, they weigh nothing, and they're a great souvenir. I'm very sad to see them decline, but maybe I'll see a resurgence in the future.
The main part of my collection began with the ones I received from my father, and other friends and family, along with blank ones I picked up traveling around. A lot of the blank ones have been and will be sent off to various places via swapping sites.
Information:
The study and collection of postcards is known as delitology, from the Greek deltos, "writing tablet, letter" and the suffix -ology. It was coined in 1945 by Professor Rendell Rhoades, after responding to a contest to create a better name for the hobby than "philocartist."