LPT: 24h AB tickets for 8,90 €* instead of 11,20€
If you don't have a DeutschlandTicket but you have a BahnCard 25 or 50 (excl. Youth BahnCard) you can buy reduced fare tickets from Deutsche Bahn / VBB for Berlin AB. They are called "Regionaltarif bis 25 km". Note: BC25/50 does not allow you to buy reduced (ermäßigte) BVG tickets!
All you need to do is to go to Deutsche Bahn (website / app / ticket machine) and purchase a ticket from your starting destination to any village just outside of Berlin C involving Regional Express (RE). Examples: Dabendorf, Nassenheide, Trebbin, Bestensee, Kremmen. I recommend you check the Regional transport map from VBB. Just make sure that it's not too far away from Berlin AB and DB shows the correct 25 km rate.
Also: these tickets are - except for AB, where you can go around wherever - for direct routes only. So if you want to go into C you have to choose a station outside of C where your actual destination is en-route. So, if you for example want to go to Königs Wusterhausen (which is in C) you have to select Bestensee as destination.