If I'm not doing missions, usually I just looking up to my trophy collection and see what's missing, and try to complete them.
And if I feel like bored, I just hijack a car and go cruising around liberty city, and "swapping" cars with other drivers in the streets, hoping to get supercars, which is pretty rare, since Niko is somewhat the "Trendsetter" for cars that exists on the streets.
Hopefully, when I encountered pigeons, I'll shoot them down (the most annoying trophy requires this) with a handgun, and there must be some cops on my tail, acting like the pigeon is theirs, and there always be more time to play.
However, I decided to concentrate in GT5 more, since I'm not the kind of guy who holds the principle "Rules are made to be broken" or something like that. As seen in my GTA gameplay style, I'm a pacifist (except if it's job-related).