We pulled a story starter from Lou Willett Stanek's book "Story Starters" as a writing exercise to do, that we'll share and discuss next time we meet.
He had given your character a key but insisted that she call before coming over. One night she was in his neighbourhood and, on a whim, she stopped by.
Feel free to switch genders around, and aim for something of short-story length (max of a couple of thousand words I'd say).