On the pause menu you can check your completion percentage. There are a number of things beyond the main story missions you must do to reach 100%. It is worthwhile to start on these early, as many of them like the hidden packages and the vehicle missions offer permanent upgrades to your stats and safehouse equipment.
Note that some story missions are permanently missable. if you move on to a new mission provider without finishing all the missions for your current one, it is possible to lock yourself out of remaining missions at several points in the game. To avoid this, prioritize missions for Luigi, Joey, Kenji and Asuka when available before visiting Salvatore or Donald.
- Complete all Story Missions
- Complete all Phone missions (4 phones)
- Complete all Off-Road Checkpoint missions (4 locations)
- Complete all TOYZ RC missions (4 locations)
- Deliver 100 passengers in Taxi
- Complete Level 12 of Paramedic
- Douse 20 fires on each island (60 total) in Firefighter
- Kill 20 targets on each island (60 total) in Vigilante
- Complete 20 Rampages
- Locate 100 Hidden Packages
- Perform 20 Unique Stunt Jumps
- Complete all 3 car lists for the Import-Export Garages