It's Teacher Appreciation Week!
I'd never discourage you from getting your teacher something nice, but one of the very nicest thing you can do for a teacher is write him or her a note about what you've learned. This goes for past as well as current teachers (which means you're not off the hook if you're an adult!).
However, the other most appreciative thing you can do for a teacher is not only appreciate teachers one week a year. Chime in on the teachers you know when you hear someone run down teachers. Support adequate school funding and good working conditions (including at town meetings and at the ballot box). And do that all 52 weeks of the year.
No one goes into teaching to get rich, to get famous, or to have an easy life. Backing our appreciation up with actions is the best appreciation we can give, year 'round.