While circular driveways are easy enough to plan and layout, It's often difficult to come up with ideas for planting and hardscaping the center area just inside the circle. Generally, most folks just go with some kind of planting displays, walkways, or even small water features.
It may be helpful to come up with a solution if you keep in mind that even though the center island is secluded from the rest of the landscaping, it should somehow tie into and be uniform with the rest of your yard, design and home. By incorporating some of the same plants, hardscape materials, or even the same materials of the home construction, it will help keep the circular driveway planter or planting area unified.
In this planter we've used the same bricks and stucco from the home to make it an extension of the home itself. While the landscaping itself didn't have much for plants and was just mostly lawn, we created a totally unique Southwest planting scheme that matches the style of the home but doesn't have any ties to the rest of the landscaping.
This design is simple in the sense of plant selection. There are five plant varieties in this small garden but it still creates impact without being too busy. The plants are of a xeriscape nature (Cherry Sage, Red Tip Yucca, Blue Salvia, Texas Sage, and a small Mexican Elder tree in the background). The Texas Sage is silver which is neutral and helps tie the other plantings together.
The boulders are of different colors which is unusual for one of my designs. I normally use the same type to help create unity. However, in this circular driveway island planter we went with what the client requested and it works well.