My father had a MASSIVE stamp collection - it's in storage waiting for me to have god knows how long to have a look through it.
I collect American militaria of all kinds, but one of the most interesting sub-collections one can build within a militaria hobby is postal history. Patriotic covers with regimental imprints are my favorite, used or unused. I really like the vividly colorful ones.
And soldiers' letters and diaries are often absolutely riveting to read. But I even like the envelopes without the contents.