One of my favorite self-challenges in GTA4 was to try an “walk” a chain explosion across one of the one-way bridges. I’d steal a car, then park it crossways at the end of a bridge to block the vehicles leaving it, then start punching my car as traffic piled up. Car catches fire, then explodes, starting the chain, and the challenge was to keep those explosions going all the way to the other side of the bridge, by slowly advancing on foot and causing more cars in the distance to pop in and ‘feed the hopper’. Get too close, and an explosion or the cops could end it, and often your luck would run out on cars appearing as well, forcing you to get closer to the still burning ones to try and streeeetch that pop-in area…
Furthest I ever got was about 3/4. Still more fun than half the game missions, and I probably blew a good 10-12 hours on that activity. Anyway, I think I discovered it because the dense traffic was annoying me.