If you can, go for global illumination over abient occlusion if you want a realistic look. Ambient occlusion is a pretty rough approximation of diffuse light from the surroundings. A proper global illumination rendering captures the same effect more accurately. You get a lot of subtle hues in the lighting which instantly make even flat single color surfaces more interesting and real.
You can also go over board with your materials, adding texture maps for roughness and normals, which vary how the surface reacts to the light. There are plenty of free materials out there that demonstrate what I mean. But good texturing is a topic all of its own, with its own tools and processes. If you start to add textures, my advice would be to keep them really, really subtle. The amount of variation you really need is a lot less than you would probably think.
If you look at this scene, I'm fairly certain all the lighting is prebaked in something like blender/maya etc.
There's also a lot of other little environment art things you can do. For instance, small bevels on sharp edges help things look more natural (look around your house and you'll notice even sharp corners aren't as sharp as you think). Adding roughness and specular lighting to materials helps a bit too, even things like wood or asphalt have some amount of reflectiveness at certain angles.