I picked up some bags of river rocks (and if you live in the appropriate area, I'm sure you could pick some up around your favorite stream or lake), some clearanced burlap that was of similar color to the stones, and e-6000 glue.
With a small dab of glue on each rock, I strategically placed them on the fabric. That same evening, I had created a lovely table centerpiece mat. (I had intended for a table runner, but I realized this took many more rocks that I thought, and I'm too cheap to go out and pick up more). Over an area of rocks that was somewhat flat, I threw on a small plant and voila - a very natural and minimalistic addition to my dining room.
And let me just say that e-6000 is a fantastic glue! I had some left after this project, so I went around the house fixing things, like my laundry hamper. This is one glue I must keep in stock.