For example they would go down a river and break down or they would erupt from volcano and so on and so forth. They record where they travel to and why, this helps understand how rocks erode or compact and how they move between sedimentary, igneous and metamorphic rock. In the end they did a comic on their journey as a rock.
Here are some samples:
I blogged about this project before - see here.
Well, there is a similar activity for the water cycle that I saw this on a blog:
I absolutely adore the water cycle simulation game. There are several free versions of this on the web, but my favorite is from the National Oceanic and Atmospheric Administration. You put up signs in different parts of your room for lake, soil, river, plants, ocean, etc. Each station has a cube to roll. Students travel through the water cycle as a drop of water.
As a TTOC this would be a fun science lesson to plan and have to use if/when you need it.