Create a full-sized version of the tiny zen gardens often sold as desktop decorations. Build a shallow sand garden around the edge of the sidewalk filled with fine, pale landscaping sand (don't use the craft sand usually placed in zen gardens, as it's too fine). Pace river rocks in the sand for visitors to arrange. This design is good for a small space, though appropriate for larger spaces if you have larger river rocks at your disposal.
Basic Rock Bed
Use river rocks to fill a basic rock garden bed for a smooth, luxurious-looking variation on a common design. Dig a shallow bed around the sidewalk and line the bottom with plastic to prevent the rocks from slipping into the dirt when it rains. Fill with a selection of rocks of a uniform color for the most striking look, or an equal number of very dark and very light rocks for contrast.
Rock Bed Patterns
Create smaller, shaped rock garden designs by digging holes in geometric shapes and lining them with a thin layer of concrete. Pepper the area around the sidewalk with these designs, or create a shaped garden attached to the sidewalk, like a lacy border.
Rocks In Cement
Extend the sidewalk on both sides with cement and press river rocks into this area to create a rock border. Find rocks with the widest, flattest planes possible and arrange them in clusters or patterns to suggest geometric shapes. Keep a damp rag on hand to wash the exposed surface of each stone and get rid of any excess concrete paste before it can harden in place to ensure the rocks' glossy shine.
River Rock Wall
If you're interested in building a small wall around your sidewalk to create an enclosed pathway, use river rocks to make it decorative. Build the wall as you would a brick wall, but use river rocks in place of bricks with traditional mortar. Use large rocks for efficiency in building, but keep small rocks on hand to fill in spots for shaping, such as corners.