Many Japanese maples are well-suited to being grown in a container. However, elevated containers - large planters or not - do make the plants they contain more vulnerable to cold temperatures just by virtue of their sides being exposed to the air. They are missing the insulating properties of all that soil mass that inground plantings enjoy. Having said that, the larger the container, the larger the soil mass around the roots, so some of the risk is mitigated. But a good rule of thumb is to include plants that are hardy at least two full zones lower than your own. That puts JM's just a little on the iffy side, unless you can provide some sort of additional winter protection.

The most important issue with any long-term container planting other than the planter/container having adequate drainage outlets is the soil it contains. Garden soil will not work - you need a soil specially formulated for containers. And for long term containers where the soil cannot be easily changed out or refreshed, it needs to be very durable and textural. And no gravel at the base -- contrary to popular belief, gravel at the bottom of a container or planter does not improve drainage but rather slows it down.

Before you decide what to plant, you may want to visit the Container Gardening forum and read some of the tutorials written by tapla - Al. He is a recognized authority on container gardening, especially container soils, and his posts are extremely informative and helpful. And look for his posts on trees in containers - you will learn a lot :-)

Lisafair,
Is there a bottom to the container?
Is it just rocks and then the natural soil underneath?
If it has a bottom to it, it is technically a container.
It is doesn't have a botom to it at all, and the "bottom" is technically the native soil under the rocks, you treat that as a raised bed.
I wouldn't plant a japanese maple in it anyway, it won't be able to handle the cold.
Since you aren't a garden person, try an evergreen, like a dwarf arborvitae, something small and plant little flowers in it every spring for color, like petunias.
Good Luck!