Answer: Unfortunately you did not include your geographic location in your question, so it is very difficult to make specific suggestions. In selecting groundcovers, you need to take into account winter hardiness and suitability to your climate, then take into account the specific growing conditions on site such as: is it sunny or shady, well drained or consistently wet or extra dry soil, is it sandy soil or clay or extremely alkaline or acidic, and so on. Based on that, you would identify plants that would thrive with minimal care so you get a durable planting that is easily sustainable over time.
Generally it is best to plant just one type of groundcover in a given area so it forms a carpet. A combination planting will tend to change over time as the one plant best suited to the location takes over and overwhelms its neighbors. Depending on the area, the groundcover could be very short just a few inches high or it can be taller -- low growing shrubs and knee high perennials are also an option.
I would caution you that gravel can become a weedy problem unless it is frequently raked, it also holds heat during sunny weather and adds nothing to the soil so it is not always a good choice for every location.
Your local county extension or cooperative extension should be able to help you analyze the growing conditions where you want ot plant and to identify plants that would be suitable to achieve the effect you envision. Keep in mind that although generally lower maintenance plantings than lawn, any groundcover will need weeding and watering at first until it becomes established.
Please check with your local store to see if they have any type of installation capability or if they recommend local landscapers. If not, you might check with neighbors who have had similar work done for recommendations.
I hope this helps you start with planning your planting project.
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