If you don't want to add more subdivisions, then all you can do is set the Smooth Angle to 180. Do this by making sure the cube is converted to mesh (Build->Convert To Mesh) and then double click the mesh to popup the dialog.
What Smooth Angle does is if the edges have an angle less than what the Smooth Angle is set to, then it'll set smoothing via normals for that edge. Naturally a cube has 90 degree angles, so the default Smoothing Angle of 45 would not apply to it. 180 is the max and it'll apply to everything. I recommend also moving the vertices of the cubes around to make it less brick-like and applying seamless textures that don't crease at the edges, but really if you want it to look better then you have to add more divisions and move vertices around. Cubes are cubes afterall. If you don't mind adding a few divisions, you can easily smooth the edges by selecting the edges and using the bevel tool to round them off.
