I love to collect rocks! A cool way to display them in your garden is to stack them. It's fun to try getting all of the various shapes and sizes to balance.
Note: You may have seen people do this at State or National parks. It is not a good practice to do out in nature. It can promote erosion and it leaves a human mark that should always be avoided. Here is a great little article about why people should avoid creating "cairns" in the wilderness.
Stop the Rock Stacking (High Country News)