To get 100% you have to complete the following:

* Complete all Main Missions.
* Complete all 10 Drug Deliveries.
* Complete all 10 Races.
* Complete all 10 Assassination missions.
* Complete all 30 car thefts.
* Collect all 10 cars for the Exotic Exports mission.
* Unlock all five friend's special abilities and complete the activities with Little Jacob, Brucie and Packie.
* Beat the computer or an online friend in all activities.
* Find all Random Characters and complete their missions (except for Jeff, Cherise, Clarence & Ivan).
* Complete all 30 Most Wanted Missions.
* Complete all 20 Vigilante Missions.
* Kill all 200 Pigeons.
* Complete all 50 Unique Jumps.