A heartfelt note is a wonderful idea for the end of the year. To make it special, write down things you appreciate about her. It might be her personality (kind, patient, caring, honest, supportive, etc.) or how she handled a specific situation. Tell her how much your son liked her and why. Take some time to think it through and then just write. The words will come to you and will mean a great deal to the teacher.
Currently, you also have a chance at prizes when you do Education.com activities, by participating in the 2010 Summer Activities Challenge: http://www.education.com/summer-activities-challenge/
The grand prize is a family vacation to Mexico! There are also great weekly prizes too.
That is a thoughtful thing to do, I have sent a note to the teacher and teacher assistant each year at the end of the school year along with a small token of appreciation.
I purchased blank cards & envelopes with a teacher theme on them, such as A+ Teacher or just whatever you can find in the greeting card isle. Write your note on the blank pages inside. I've always kept mine brief. Something like: Thanks for a great year! We appreciate all your hard work and support that you've provided and it will not be forgotten. Thanks again and have a wonderful summer! Sign the note with both your name and child's name.
Now include a small token, if you know the teacher well, then you may want to personalize your gift, if not, just include a stationary pack of paper, envelopes and pens.
I have learned that my daughter's teacher this year collects angels, so at Christmas, we purchased her a beautiful, homemade angel ornament that we will give to her at the end of the school year.
Little tokens like this lets the teacher know that he/she is appreciated and she won't likely forget your child either.