Volunteering in the school is great, just expect the school to do a background check on you before you can actually volunteer. Most schools/districts have tightened their security procedures in recent years.
A tote bag would be great, as would stickers, colored pens, art supplies (craft foam, google eyes, magnetic strips, pipe cleaners, etc.) as most teachers buy these items out of their own money.
I would hesitate to send snacks for the students because of the rise in food allergies (nuts, milk, red food dye).
Gift cards are appreciated. But above all a heart felt letter thanking them for the work they have done with your student is great, people are quick to complain but slow to show appreciation.