There is nothing to do once you finish all the main missions.
Period. Please dispute this if you wish.
Strangers and Freaks are horrible side missions and they ruin the good impressions you had with the main missions. Everyone you interact with is crazy and the missions they give you are not fun in any way whatsoever. Run along Vespucci Beach? Target Practice? Seriously?
Miscellaneous activities are boring. Tennis is the only decent sport to play but gets old fast. Collecting stuff needs no explanation, anyone who has tried knows how much of a chore it is. Random events? Oh how fun and detailed they are, not. Flight school and races are just terrible.
Doing your own thing is fun for a while, but gets boring. Yes, go to a military base and hijack a fighter jet or tank, then fight with the cops at 6 stars. Fun the first couple of times, boring afterwards. Buying stuff? Yeah sure, but it won't make the game any more interesting.
The most exciting thing for me now is waiting for the Pisswasser stock to hopefully increase as high as anticipated by Monday, that's how boring this game is once you complete the main missions. And even if it does go that high, what then? Buy the Country Club for $150M to play golf? Give me a break.
No seriously, what is there to do in single player once you beat the main missions?
Well since your bored of this game now and have also said you think online will be sh*t, you might as well f*ck off and play another game pester another forum. Everyone's a winner