Before explaining the activity it is important to note i am using graphical enhancing mods, my population and variety settings are maxed, i have 0.5 increased Peds & Traffic as well
My favorite activity is to just drive around and explore, i thought after playing for years i had seen all of GTA but you really haven't. GTA has awesome attention to detail and many smalls things to notice, the feeling of driving around and soaking in the views is the best. GTA truly is beautiful depending on what your computer can handle. I suggest finding some mods including realistic driving mods, graphics mods, turn your population setting up and population variety up then just go out on a drive. There are many mods for scenarios too which are always fun to explore and add variety such as Forests of san andreas for example, or a concert in los santos, they can also be used in conjunction with on duty work too