here are just a few things:

100% complete
weapon skill 100%
driving skill 100%
flying skill 100%
bike skill 100%
boat skill 100%
cycling skill 100%
respect 100%
all houses
all schools completed
maximum cash
all girls 100%
70/70 unique stunts
100/100 tags
50/50 snapshots
50/50 oysters
50/50 horseshoes
paramadic, vigilante, fire fighting, pimping, taxi driving completed
all side misions completed
all assets accquired
all special outfits
all gang territory
3 S.W.A.T. tanks in Doherty garage
4 S.W.A.T. tanks in verdant meadows hangar