These solar post cap lights by SohoDecor are a terrific idea. Four inches on each side, they fit perfectly over most wooden fence posts and can be placed in whatever pattern or at whatever distance you desire. They're available at Amazon.com
Amazon.com also carries several similarly inspired alternatives
Solar post cap lights last for many years without replacement. The flexibility of being able to place them and then move them at a later date - without having to worry about electrical supply - is a tremendous feature. You can have light at the far corners of your yard or along the edge of your deck, without the trouble of hiring an electrician.
US $114.97
