The rule of geocaching states that if you trade an item from the geocache you should replace it with something of equal or better value. Trading items range from childrens toys, keychains, small tools, games etc.

There is one exception that has an ID code and is still considered trading swag, known as PathTags. These microcoin-sized items (1" diameter) have a small hole in them and on the back a unique ID code.

Even though they look like small geocoins they are not. If you find one, you can keep it. Trade for equal or better like any other trading item. Besides, Pathtags are collectable!

You can collect, trade and track your Pathtags at Pathtags.com.

Some more information you can find on Wiki/Pathtag and What Are Pathtags?