Side yards that are long and narrow present a design challenge. Are you, yourself struggling with this challenge?
Do you often find yourself cutting through the side yard in question (that is, the left side yard or the right side yard) on a practical mission? For example, if you had to push a garden cart filled with compost from the backyard to the front yard, would you tend to cut through this particular area? If so, it behooves you to build a no-nonsense walkway through the area, a surface that you will not be tripping on.
A wide, attractive walkway running up and down a narrow side yard can also serve a function similar to that of a patio. If, by contrast, you do not cut through the area often, you have some less formal (also cheaper and easier) options at your disposal. For example, you could lay down garden stepping stones to create an informal path that wends its way through the side yard. Curved pathways are more attractive than straight ones (a straight path will reinforce how tunnel-like the side yard is, which is precisely what you wish to avoid) and are a good choice where aesthetic concerns are paramount. Plant a ground cover such as creeping thyme between the stepping stones, to add visual interest.
Alternatively, you may choose not to have a well-defined path at all. You can suppress weed plants in the area by laying down weed fabric, covered by mulch. As long as the area is not heavily trafficked, you can also grow a nice green lawn (although this option does entail regular maintenance, so be prepared for extra work). If the area is shaded, select a shade-tolerant type, such as tall fescue grass.