Most teachers keep at least a few mementos from special students. Some keep scrapbooks while others collect things, like ornaments. One nice thing about giving a seasonal ornament to a teacher is that it can be boxed up with the rest of the holiday decorations and become part of a yearly collection. Ornaments are a perfect size, small but not insignificant. Better yet, crystal, brass and silver ornaments can be engraved with the year and your child's name as a permanent reminder.
If this is too formal, your child can fashion an ornament masterpiece for his favorite teacher using a three inch Styrofoam ball, glue and glitter. Many retired educators find that handmade and heartfelt sentiments are the most touching anyway.