We show a relation of the classical nova outbursts and the activity of post-novae. A variety of the optical activities of post-novae exists. Also large changes of the type of activity in the decades after the classical nova outburst are observed in some of them. Although the ensemble of systems in this analysis is incomplete, the similarities in the decaying branches of the outbursts in some post-novae are the features favoring the dwarf nova outbursts. The features consistent with the dwarf nova outbursts appear mainly in the later phases (years, decades) of the gradual decay of the mean luminosity of some post-novae. This suggests that they contain the accretion disks which were able to switch from the hot (ionized) state to the thermal-viscous instability (TVI) regime. Such a TVI of the disk can appear in some post-novae even during the first century after the classical nova outburst (often sooner after the classical nova outburst in cataclysmic variables with longer orbital periods). Activity of the mass-donating star in the brightness variations of some post-novae can be caused by the appearance and changes of the position of the active regions on the donor.