The way I make money fast is that I usually do CEO missions like "Headhunter" and "Sightseer" you can make around $25,000 when doing those missions. Also Client Jobs from the Terrorbyte but I should let you know you need to have Nightclub, Garage and the Terrorbyte itself in order to do the Cilent Jobs. They pay out like $30,000 and the ones I recommened doing are "Targeted Data" and "Diamond Shopping"
For Businesses I usually stick with the Acid Lab Business because since it was added last year with the Drug Wars DLC you can make a lot money and its the better business that is solo friendly to make money. Besides you can actually customize your delivery bike. The only way to get it is to do all the "First Dose" Drug War Missions and you'll get the Brickade 6x6 for free instead of having to pay $1,450,000 from Warstock just to have it.