I don't believe there's an afterlife, or at least not one with any kind of consciousness. We do go back to nature though, in the sense that we decompose and the molecules in our bodies become parts of other things. I'd like to have a tree planted over me, too.
If I became a ghost (much to my surprise and annoyance), I think I'd avenge myself by boring people. It's never clear what a chain-clanging, moaning ghost can do to someone, other than scare them, and people eventually learn to tune out scary noises that mean nothing. We all witness at least half a dozen "murders" a week over various media, and then we forget them. We're totally desensitized. But things that are boring actually get worse with repetition. I think I'd read to people from complicated laws regulating insurance. I'd use just enough volume and pitch variation that they can't ignore me. Like, "Tonight we'll be reading from a 1932 New Zealand law about insurance for watercraft traversing lakes and rivers, but excluding those primarily designed for seagoing use, sung in the style of Betty Boop. Part 93."