I've won several full games of bowling, beat the QUB3D high score, did all the assassinations and drug deliveries. I even have the Bradygames guide, but didn't start using it at first because I didn't wanna spoil anything in the story and wanted to find what I could find for myself.
I guarantee I finished all the main story missions, the drug deliveries, the vigilante, the most wanted, the exotic exports, the races, car thefts (via text msg), assassinations, pigeons, and stunt jumps. I made a checklist and went through to make sure I took Brucie, Jacob and Packie to all the activities each has available to do, and beat them all at their activities.
And now I'm going back to all the locations where the random characters are located to double check and make sure I finished all those missions, though I definitely remember finishing them all (though whether or not I had auto-save on at the time, I can't be completely positive... it was about a year ago when I last played)
At this point, I'm pretty clueless as to what I must've forgotten. All's I know is it has to be something worth 1.25%