New job is too easy. How do I bullshit?

I started a new job three weeks ago and I’m realizing that I’m overqualified. I’ve been knocking tasks out quickly and sit around looking for busy work. I work next to a guy that believes that he’s my boss but I don’t think he is. If he see’s me on my phone or anything then he just gives me random shit to do. I’m pretty sure he can see my computer screen as well. So like… what do I do when I’m done and bored as shit???