If your side yard is a road between a front yard and a back one, you can use it as connective means. For better result arrange it in the same style as these two yards, set plants that already grow there. A useful tip: make a front yard visible from a back one and vice versa, thus, you will create an illusion of a big garden.
A side yard should not be bare. One of the most successful side yard landscaping ideas is using bamboo which will provide a nice view and additional shade. It could contrast well with fern in pots, for example. In addition, bamboo does not require much water. By the way, potted plants will be good for your side yard if you have enough space there. You can also attach the pots to the walls; in this case they will decorate not only your yard, but also house facade.
If your side yard is hidden behind a fence, nobody will observe you. So, you can do a working area there: it could be a place for artistry or potting. If there is a slope in your side yard, you should level it taking excessive ground away. Set vertical plants there to make the ground more horizontal visually. By the way, if space permits, you can use your side yard for constructing a patio there. If you decide to grow fruit and vegetables in your side yard, pay your attention if there is enough light for them. Sunlight must be present there during at least eight hours in summer. We wish you success with all your side yard ideas!