Objective: A Pleasant Day
Go downstairs to the kitchen as instructed then eat your pancakes. Next, head outside to pick up the newspaper then drive around town with your friendly policeman. Unfortunately, you'll be very restricted from driving and you won't be able to run over pedestrians (like what you really want to do)
After reaching your destination, walk a bit to pick up the health pickup then do a photo-op. After the scene, drive around until Kinzie successfully contacts you. Head to the waypoint and start shooting at the cops. Keep killing all three waves of them until Kinzie instructs you to get the rocket launcher from the cat fountain. The ammo is infinite so go all-out and unleash hell.
Finally after wrecking enough havoc, kill the sheriff. He uses a shotgun so you have to sprint away from him when he jumps to your position. Since you're using a rocket launcher, aim a little lower so the rocket hits the ground and deal splash damage to him. Keep doing this until you defeat him. Watch the following scenes and the mission will be complete.