Complete all levels of Insurance Fraud
In Insurance Fraud, you must take dives in front of cars to rack up imaginary 'hospital bills'. You must get a certain amount of money to beat each level. This is probably the hardest activity for most people, but can be made a lot easier.

When you start, make sure the place you are headed to has a highway. If not, quit then restart until it gives you an area with a highway. Go to the highway to find big rigs all over. Go in front of one and hit [ Register or Signin to view external links. ] right before it hits you. You will go flying. Now, while you are flying through the air, try to use [ Register or Signin to view external links. ] to veer into its path or the path of another truck. You can make 150k off a single dive.

There are 2 Insurance Fraud activities with 6 levels apiece. I recommend doing this in co-op so you can work towards the "Confidence Men" achievement as well as "Crime Lord".

This achievement rewards 15G after completion.