I never beat Arkham City either. Any open-world game turns into Grand Theft Auto and I just run around causing as much chaos as possible.
Most of the time I've played Watch Dogs I drive around and crash as many times as possible, carjack people, toss bombs everywhere, and throw my spider bot at everything. Then, when everybody's good and pissed at me, I run into the nearest bar and start beating the shit out of everybody in the room with my bare hands until I get overpowered and die.