There are a number of ways to make money in GTA Online that require skill, finesse and, sometimes, simply signing in. Success, however, is often determined by the folk (read: infuriating griefers) you're forced to rub shoulders with in public forums.
With this in mind, HKH191's Executive Business mod takes GTA Online's CEO business ventures offline, and into its singleplayer mode. Griefing problem solved.
Complete with an office assistant as per GTA Online, here's some moving pictures:
At present, the Executive Business mod lets Franklin, Michael and/or Trevor purchase the Arcadius Office, the Lombank Office or the Maze Bank West Office—with gun lockers and money vault as optional extras.
Likewise, vehicle warehouses, aircraft storage lots and multi-car garages come with comparable price tags, and offer GTA Online-like missions. The latter includes the likes of supply runs, sourcing and selling vehicles, asset recovery, and Convoy details wherein players must chase and destroy armoured escorts for big money.
More on all of that can be found on the Executive Business mod's GTA 5-Mods page. Modder HKH191 is also responsible for Lester's Bribes, Biker Business and Weapon/Vehicle Trader—all of which bring parts of GTA Online to Grand Theft Auto 5's singleplayer world.