Plant a small garden around the sprinkler valves. Use an attractive, low ground cover or mulch to keep the ground neat and free of weeds. Plant a tall, soft perennial plant in front of the valves. A non-woody plant will allow you to reach behind to use the valves. Try hostas, which grow, large green leaves that will hide valves easily. The leaves can be pulled aside while you access the valves.
Design a rock garden around the sprinkle valves. Put a permanent mulch down to create the base for the garden, and cover it with gravel. Set larger rocks around the garden. Use a large, upright rock in front of the valve. Set the rock several feet in front of the valves, so you can see behind it and reach around it, but the valves will still be obscured from a distance. Plant a few plants into the rock garden for color.
Use a structure to hide the valves, such as a large clay pot or flower box. For higher valves, try a bench, fence or dog house. Make sure you can easily access the valves without having to move heavy structures.