Optical activity of cis coordination compounds
It is not always as easy to see from a standard octahedral depiction. Optical activity in octahedral complexes can be understood better, if you draw the octahedron as looking onto a triangular face rather than along an edge; this results in a hexagonal arrangement of ligands around the central atom with ligands alternately pointing into and out of the paper plane.

If we do that with $\ce{[Co(en)2Cl2]}$, we get the following two enantiomers which you cannot convert into each other:



The complex is chiral because the $\ce{en}$ ligands necessarily cross the paper plane and can do so either in a right-turn screw motion or in a left-turn screw motion.