There is an interesting glitch involving a couple of swing sets in a park in the Firefly Projects. The park is located in the south section of Broker, just a bit above the west-end of the freeway to Algonquin. Get a boxy car and some Molotov cocktails and make your way to this park.
Nudge one of the swing set posts (the north-most swing set seems to work best) with the front or backend of your vehicle and the car may be sent spiralling off into the distance. Since it is likely that Niko will crash through the windshield as the car soars through the air, holding a Molotov cocktail before triggering the glitch will prevent this. Only Molotovs are suitable, however, since they do not have a timed detonation like grenades. This video is from our unofficial game guide. You can view the full guide here