Write a heartfelt letter
I have talked to a lot of teachers over the years. They tend to forget gifts like mugs, magnets and knick knacks. Every one of them has a story about a letter or card they received that was so heartfelt that they have kept it forever.
I can see why this would be a big deal to a teacher. They have an often thankless job, underpaid and often running on financial constraints. To hear they they have touched a student in a way that made them enjoy learning would be so worth it to hear.