SP only, I never play V online. Here goes....
* I haven't purchased all the businesses because the business system is so flawed and it's very linear too.
* I haven't explored underwater that much, because I did not really see much there that was exciting or interesting.
* I haven't played tennis or golf. The lack of crime side missions made me not bother with sports.
* I have not done yoga apart from the story mission. As if that was ever gonna happen.
* I did not bother hunting, the wildlife was so mediocre that I was put off from it.
* I did not bother to finish all of the Strangers and Freaks missions, most of them were so boring and uninteresting.
* I did not bother finish the game on the two other endings, I was happy with killing Trevor and leaving it at that.