Yes, gift cards are great! But they can get expensive. And we like all sorts of thoughtful things. This year I was so excited to get anything...it was so much fun.
I got lots of flowers (potted are best), a sports drinking bottle (love it), notepads, lots of sweets, and can't remember what else.
I made a flower pot for the teacher I work with (I got the idea from another mom from here). I used a terra cotta pot and wrote her name, class and year around the top and in the middle I wrote "Thank you for helping us grow" in cute letters. Then I had each student dip their thumb in green paint and stamp it on the pot. I wrote their name and turned the print into a bug. Then I planted pretty flowers. You could get smaller pots and have your son stamp his hand on it and say "Thank you for helping me grow." For this I used a marker that was for outside use and on terra cotta and same with the paint.
I made fresh salsa for my kids' teachers and added a little baggie of chips. They LOVED it...I think partly because we get so much sweets that it was a nice change.