I just finished the game for 100%. Still, I'm missing some stuff, like a bulletproof Patriot and some kind of a plaque for my warehouse. Here's what I did:
- Finished the story mode
- Destroyed the 100 cameras
- Jumped 30 USJs
- Got medals on all rampages
- Got medals on all races
- Got medals on all timetrials
- Got medals on other oddjobs
- Got medals in Liberty City Gun Club
- Found all 8 random characters and finished their missions
- Got all 21 safehouses
- Found all 80 dealers
- Got all 8 rewards for the warehouse (wooden spoon, titanium briefcase, golden binoculars, platinum syringe, jeweled bong, jeweled key to the city, silver safe, diamond pillbox)
So, just for the reference, ^ here's the list of stuff needed to get 100% ^. Not sure about the 8 random characters, though. Also, cheating does not drop your percentage, at least not after you got 100%.