A one-shot character

How about being one of those characters who showed up as part of the crew for one episode, often as a love interest for one of the regulars? Dr. Helen Noel (the psychiatrist Kirk was programmed to fall in love with) from "Dagger of the Mind" for instance, or that blonde kung fu girl who made out with Chekhov on the planet of the Ken Dolls in "The Apple." Or one of the guys like Ensign Garrovick from the space vampire story ("Obsession") if you want to take things in another direction.