The urban landscape is dotted with small businesses, nearly all of which are fronts for illegal rackets that you can take over and extort money from. Usually this involves terrorizing a shop owner into paying protection by smashing up his business or his face (or just talking to him, if you're respected). He'll then unlock the back door, and you can shoot your way through whatever thugs are guarding the prostitution, gambling or smuggling rackets behind it. Intimidate or buy out the racket boss, and you've won another territory for the Corleones.
This gets repetitive after a while, and the enemies in each racket type tend to attack in the same predictable patterns. Even so, single-handedly seizing rival operations and watching your influence spread across the map is uniquely satisfying.