If you mean you finished the main storyline but not "everything" for 100% then there is plenty left to do:
- Assassination missions (from Alderney payphone)
- Little Jacob's delivery jobs (chose from your cell phone options when calling Jacob)
- Brucie's car delivery jobs (from e-mail)
- Brucie's street races (from cell phone)
- Stevie's car delivery jobs (from text messages)
- killing top 10 "most wanted" criminals on each island (from police car computer),
- vigilante missions (from police car computer)
- taking remaining friends out to various activities (becomes boring once you are too good at them though)
- if you didn't already find them all, random characters which show up as green icons on your radar if you are in their area
- finding and killing all 200 pigeons
- finding and doing all 50 stunt jumps
If you mean you've done all of the above and already completed 100% then just explore any areas or things you didn't previously do, fly/boat around if you haven't done much of that, or if you like the violence or challenge, cause chaos around town and fight out of trouble.
Last idea for endless variety until you get bored with the game, play online with friends.