I won't fully know what to do with my boredom time when playing GTAV, but I know what I did in GTAIV for the boredom.
1. Walk people off "cliffs" around the city. You don't use the push attack, you just walk into people and push them by walking, until they go splat at the bottom of a "cliff."
2. Rail sliding motorcycles on certain ledges and stairways.
3. Riding back and forth on the western edge of the second island parallel to the Statue of Happiness. You could run over endless amounts of people for hours and hours.
4. Causing traffic jams by spawning cheat code boats onto the freeways. This results in a lot of splats.
I'm sure I will be able to repeat a lot of the same boredom cures in GTAV, but I'm really really hoping I can push people off of the top of Mount Chiliad and watch them fall all the way down until they splat.