You know what I like to do? I like to respond to Craigslist "for sale" ads with critiques on the items listed. This one guy was selling so-called "mid-century side tables" when, in fact, they were not mid-century. I would place the items as 80's vintage, and I wrote to tell him this. I included examples of what mid-century items were, as opposed to items of a later date. These people whom I feel the need to correct always respond with attitude.
I also like to inquire about cars for sale; I'll ask a series of mundane questions and when they're answered, I'll then ask another series of clarifying questions. I have no intention of buying a car, but I like to bug people who are selling them.
When in grocery stores, one of my favorite things to do is to bother the employees. I'll order, say, one pound of ground chicken. Well, most people don't care if it's an ounce one way or the other, but I do. I want my order to be exact and I'll make sure the employee does just that. Sometimes, after a series of scoops to adjust the weight according to my request, I'll either cancel the order or say, "Oh never mind, make it two pounds instead." I love seeing the frustration in the person's eyes, and I love how they can't really do anything about it. I also tend to bother the person bagging my groceries and make their life a living hell. I like my groceries bagged in a certain way, but I won't tell the person until the bag is half full. Then I'll demand that they re-bag it accordingly.