While it’s much girlier than my normal taste, I really like the idea of sprucing up curtains with trim. It makes a big impact with very little expense or effort.
Above, Love Maegan has a good tutorial for black tassel fringe curtains, featuring this trim. A variation is this bright pom-pom trim, from a post on This is Glamorous (room designed by Lavender & Plum). There’s no tutorial, but I did find this similar trim with a quick Google search. Not pictured: put trim on the bottom of short curtains.