Here is a list of everything you need in order to get 100% completion in GTA 5.
Note that you can go for 100% at any part of the game. Everything you don't do can be completed after the story and you can't miss anything for 100% completion.
Story Missions (69) - Complete all Story Missions, including Lester's Assassination Missions which are only available to Franklin. (50% completion)
Hobbies and Pastimes - Complete the following hobbies and pastimes: (10% completion)
Lap Dance at Strip Club
Shooting Range (must score at least a Bronze in 3 rounds with each weapon type)
Parachuting
Flight School (must score at least a bronze in all 12 lessons)
Sports (Tennis, Golf, Darts and 3 Triathlon races)
Strangers and Freaks - Complete the following Strangers and Freaks missions. (10% completion)
Beverly paparazzo missions
Hao (shift work will open street races)
Barry (Only Franklin's Missions)
Fanatic (Only Franklin's Mission)
Dom (all 4 extreme missions)
Omega (collect and return all 50 spaceship parts)
Dreyfuss (This will open up after collecting all 50 letter scraps
Random Events - Complete 14 Random Events (15% completion)
Miscellaneous (15% completion)
Must purchase 5 money-generating properties
Must purchase a vehicle via the web
Visit the cinema
Walk and play with Chop
Use a prostitute
Booty Call
And 4 Friend Activities (strip club, bar, darts and a movie)
Total Completion: 100%
Note: If you go to your Rockstar social club profile and click "View Stats", you can see all of your GTA 5 character stats, including how much of the game you have completed.