This is one of the few collecting fields which is still quite inexpensive and yet there are literally thousands of different items to collect. It can be as narrowly defined as say "TV characters", or, like my collection, it may include almost any type of play money. You are limited only by your imagination!
As an aside, "Ephemera, exploring the world of old paper" posted a nice article about the hobby: "
An Interview with Play Money Collector Jack Phillips".