Honestly, we always avoid HOHO buses. You end up stuck in traffic, barely able to glimpse anything, and spend the day with only other tourists with no flavor of the city. A full day Athens transport pass for metro/tram/buses is only about 5EUR. the metro can get you into Athens from the port and the major sites are not only within a few blocks of each other, but also not easily visible from the streets the HOHO buses need to use so you will see little of the true highlights of Athens on them.
Better to use public transport and walk and really experience the city