I say go for it. I drop by my teachers' office all the time (not even during office hours). I'm not sure how open your school is to close relationships between professors and students... mine is very relaxed. I drop by to chat, make jokes, or have a laugh. I think your teacher would appreciate you taking the time to say goodbye. In my experience, teachers love students who show interest outside the classroom (it doesn't necessarily have to be class related. It makes the teacher feel like they've done their job)