Here's my new favorite: Mrs. Spitzer's Garden by Edith Pattou. This book is so subtle, I didn't even recognize Pattou's metaphor about the classroom and the garden until the second-to-last page. When I did, my eyes welled up with tears. A good teacher truly is planting a garden each year, and this book will demonstrate to your child's teacher how much she means to you and your child.
Other picture book possibilities with similar themes include Patricia Polacco's, Thank you, Mr. Falker, Judy Finchler's Miss Malarkey books, Harry Allard's brilliant Miss Nelson books, and Michael Garland's rather crazy Miss Smith books.
And if you are looking for some novels with great teachers in them:
Frindle by Andrew Clements
The Fabled Fourth Graders of Aesop Elementary School by Candace Fleming
The Wednesday Wars by Gary Schmidt
All classics, perfect to tell your child's teacher how amazing you think they are.
Or how amazing you wish they were: "See! Read this book. Look what you could teach my child if you only tried!"
I'm just kidding. You should not say that. They are all trying. And they all deserve presents. Not coffee mugs or boxes of fake chocolate, but books!