I have 100% and I'm missing a ton of stuff. I'm guessing that the only things that count are:
Missions and Strongholds for the main story Activities CDs + Tags
My brother and I didn't do many diversions, and since we never completed them all, I'm positive they don't count. Obviously jumps and barnstorming as well since we don't have them all. My brother is also missing several cars, homies (never did muggings for Jane), shops, you name it.
Basically we just played the main activities and story co-op, and then we decided to find all the tags and CDs for the achievements. As soon as we completed those, we got 100%.
Prologue - 4 Missions, 1 Stronghold Mission: Jailbreak Mission: Appointed Defender Mission: Down Payment Mission: Three Kings Stronghold: Stilwater Caverns
The Brotherhood - 11 Missions, 4 Strongholds Mission: First Impressions Mission: Reunion Tour Mission: Waste Not Want Not Mission: Red Asphalt Mission: Bank Error in Your Favor Mission: Thank You and Goodnight! Mission: Retribution Mission: Jail Bait Mission: The Enemy of my Enemy Mission: The Siege Mission: Showdown Stronghold: Sommerset Apartments Stronghold: Wardill Airport Hangars Stronghold: Imperial Square Pagodas Stronghold: Poseidon Alley Docks
The Ronin - 11 Missions, 4 Strongholds Mission: Saint's Seven Mission: Laundry Day Mission: Road Rage Mission: Bleeding Out Mission: Orange Threat Level Mission: Kanto Connection Mission: Visiting Hours Mission: Room Service Mission: Rest In Peace Mission: Good D Mission: One Man's Junk… Stronghold: Humbolt Park Science Museum Stronghold: Amberbrook Museum Pier Stronghold: New Hennequet Rec Center Stronghold: Suburbs Strip Club
Sons of Samedi - 11 Missions, 4 Strongholds Mission: Got Dust, Will Travel Mission: File in the Cake Mission: Airborne Assault Mission: Veteran Child Mission: Burning Down the House Mission: Bad Trip Mission: Bonding Experience Mission: Riot Control Mission: Eternal Sunshine Mission: Assault on Precinct 31 Mission: The Shopping Maul Stronghold: Elysian Fields Trailer Park Stronghold: Bavogian Plaza Drug Labs Stronghold: Stilwater University Student Union Stronghold: Sunnyvale Gardens Fishing Dock
Epilogue - 4 Missions, 1 Stronghold Mission: Picking a Fight Mission: Pyramid Scheme Mission: Salting the Earth…Again Mission: …And A Better Life Stronghold: Rounds Square Shopping Center
Extras - 1 Mission Mission: Revelation
Activities Total: 33
Demolition Derby x1
Crowd Control x2 Drug Trafficking x2 Escort x2 Fight Club x2 Fuzz x 2 Heli Assault x2 Insurance Fraud x2 Mayhem x2 Septic Avenger x 2 Snatch x2 Trail Blazing x2