I agree with the golf way, if you take a 9 hole game you get a nice 2000 Rp and the strenght ofcourse.
Also I do everytime when I go online the Boneyard survival, I do it alone, takes about 25 min. to get to wave 10.
Nice cash rewards en also again some nice Rp
If you want to know how to survive alone, just stand behinde the porto cabin on the right site and the enemys will run in to your line of fire.