I'm going to leave a small dent on someone's car bonnet
Why do you want do dent their car's hood?
I'm just bored.
That is really disrespectful to damage someone's car like that. You should be respectful of other's property.
I didn't hurt anyone, so seriously I'm just bored.
If you are bored, then find any other number of ways to satiate your excitement. Go out and about with your friends or visit some family. Play a video game, see a movie or take up a sport, literally anything else but damaging someone's property.