Gift cards are always great and rarely wasted!
Please please please avoid candles, mugs, scented hand lotions, trinkets, etc. Honestly, when I cleaned out my office before taking another job I threw out boxes of that stuff. Most of my teacher friends do the same.
Some of my favorite "thank you" gifts (honest) were hand-written notes from my students and their art work. I have framed many of them and they cover my office as I type! Each one reminds me of a child and their story.
One particulary difficult child (severe emotionality and learning needs) told his mother that I needed a "day off" after the year I had with him. His parents (wealthy) "surprised" me with a spa day prescheduled two weeks before my wedding. It was overwhelmingly thoughtful. Not typical, but what made it special was the fact that it was the child's idea and that it reflected the relationship we had.