Why? Because trim work is my favorite! My goal is to trim every window in my house... which, because we have a lot of big windows, is a LOT of trim :)
There are a lot of styles in which you could trim out a window. Mine is a Cambridge/Craftsman hybrid with an 8" top casing and a 1/4" fillet. (The lighting in this room is tough for picts, so I attached a picture from online below to give you an idea what it looks like.)
Total cost: About $30 (not including the paint and caulk which I already had)
Total time: a few hours
Total impact: Huge! This is a simple way to add architectural impact to your home.