If you want to grind a simple online mission Pot Shots is really easy if you use a car to stop the van from crossing the bridge in the beginning of the mission. On Hard you get like 1400 RP and $7000 and when you get decent at it, you can complete in like 4 minutes, so that's like over 20k per hour. I know it's nowhere near the old Criminal Records method, but it's better than nothing and is pretty easy to complete on your own.
Hardest Achievements: Seriously…, Mile High Club
Yes, I played Avatar, I’m not proud of it, but whatever, if you’re on this site, chances are you did too, or have seriously considered doing it, so :-P