You can buy a gift certificate to teacherspayteachers.com where teachers share items that they have used with success in their classroom. My "cart" has $27.00 sitting in it as we speak! It is a fun resource.
Gift cards. This is kinda two-fold. I love to get gift cards in $10 increments for places kids love (iTunes, movies, food places, etc.) that I can use as rewards for my students. I like to do monthly drawings for the kids if they have done all their homework or other criteria for the month. Then as a teacher I love to get gift cards to amazon.com to buy things for my classroom, books, etc.
Wine (OK this might not be super appropriate for every teacher and/or school situation but you know some teachers would love it...and yes, I do love Apothic Red..thanks! Ha!) I was thinking that wine or food for teachers would be a great gift. (Of course, not to drink at work but at home after a long week with all the kiddos!) Even buying the teachers lunch for a random Friday at school or something is always a big hit...you can get other parents involved and host a huge salad bar in the teacher's lounge...always a big hit!
And the best gift you can give is to work with your teacher and student to make sure your student has a great year! Help your student stay organized and prepared each day.
And if you are a spouse of a teacher...you should definitely buy her a new watch for the school year like this one above. (hint, hint!)