As I understand it, Eberswalde lies outside of the coverage area of an ABC day pass. I will be making a round trip to Eberswalde on a Sunday, but I will also be making trips just within zones A and B that same day. Is there a single ticket I can purchase for all of my travel that day, or do I need to purchase an Eberswalde round trip ticket as well as an AB day pass?
Buy a Brandenburg-Berlin-Ticket for 29€ at a ticket machine. There are no time restrictions on Sundays. You may only use regional trains (RE, RB, S). The ticket is valid for S- and U-bahns, streetcars and busses in Berlin, too, and you may use the public transport in Eberswalde.
If you want to have it cheaper buy a roundtrip ticket to Eberswalde for 11.40€. Then you need an additional day ticket (Tageskarte) for zones AB in Berlin for 7€.
It could well be that a Tageskarte to Eberswalde includes transport within Berlin. The VBB Tageskarte I used for a day trip to Eisenhüttenstadt was also valid for the U- and S-Bahn.