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.
If you go to an Internet café and enters the URL www.whattheydonotwantyoutoknow.com you will get to a page with maps of all weapon pickups, health, armor, vehicle locations, unique jumps and pigeon locations.
If you call 911 on your phone you can select if you need an ambulance, police car or a fire truck. Make your selection and wait for them to arrive so you can steal the vehicle. If you get into the ambulance you will get a small health boost and if you get into the police car you will get 5 shotgun shells.
Get Claude's outfit:
If you choose the kill Playboy X you will take over his safehouse. If you change clothes in that safehouse you can choose to wear the outfit Claude (the main character in GTA III) was wearing. Black jacket, green pants and black/white shoes.
Get to Happiness Island. Run up to the base of the statue to find a shirt on the ground. Walk over it to put it on. It has a picture of the statue and the text Happiness is... on the front and the text ...land on the back.
The heart of the Statue:
Get a helicopter and fly to Happiness Island. Jump out of the helicopter at the top of the statue's base so you'll land by it's feet. You can not take the stairs this high. Look at the sign next to the door. It says No Hidden Content Here. Enter the door and climb the ladder inside the statue. You will find a big, red, beating heart held in place by chains.
Head inside the old, abandoned Casino where the last mission takes place, at the north end of Alderney. Look at one of the trash piles on the ground to find a purple dildo sticking out.
Across the road from Bank of Liberty you can find a house covered in graffitti. Some of the artwork is from old GTA titles.
Across the street from the museum in Algonquin you can find a portrait painter. The portraits are of old GTA characters.
Old GTA character's names graffitti:
Go inside one of the pink houses near Manny's community center. Go to the back of the inside and look at the wall. You will find graffitti with names from a lot of old GTA characters, like Claude, Tommy, Toni, etc.
OG Loc CD:
In your Bohan safehouse you can find a CD by OG Loc from San Andreas on the floor.
Golf Course Land Marks:
At the mini-golf course at Firefly Island you can find a small version of the giant chicken from San Andreas, the lighthouse from the GTA III version of Liberty City, the Watts Towers from Los Santos and the Ocean View Hotel from GTA: Vice City.