As you have heard, gift cards are a favorite. However, if your kid is in middle or high school, that can add up pretty quickly. Other fun gifts that I've gotten over the years include gel pens (great for grading!), chocolate, and a simple letter written by the child wishing me a nice break and letting me know they value my teaching.
I may be the odd one out, but I really prefer consumable gifts. As a middle school teacher, I sometimes end up overwhelmed with little trinkets and keepsakes from my 100+ students. And year after year...eventually you just can't keep them all and that is really hard. The notes I keep in my files, but stuffed animals, ornaments, etc. can be just too much sometimes.
Have fun and remember that the big thing is just letting the teacher know they are valued. It doesn't have to be anything big.