If you search the internet for "Athens City Pass" you will get plenty of offers. Which one to choose depends on what you want to do and how much time you have.
The "Skip the Line" argument often displayed in these offers is nothing more than the fact of having bought your ticket in advance and not having to take it on the spot. You will not have any privileged access with this "Skip the line".
In October, only the Acropolis site itself could justify buying your ticket in advance. You can just as well buy your tickets for the other sites or museums as you go, especially since although most of the sites are within walking distance from eachother , you probably won't have the time or want to visit all the sites included in a Pass.
It's on the official website below that you will have the best offers (from a price point of view).
EU citizens under the age of 25 have free entry to all official sites. EU Senior Citizens over 65 get 50% off.
Note that if you are entitled to a reduced rate or free admission, you will have to go to the ticket office to prove your age with an ID, so the "skip the line" advantage is definitely no longer of any interest.