One year I gave each volunteer a small plastic flower pot, with a small ziploc bag of soil, and a packet of seeds. Enclosed was a message something like: "Thank you for helping our school grow!" or "Volunteers like you help our school grow!"
We are a nursing home and our volunteers help with a variety of tasks. Some volunteers help residents with meal, some assist the hairdresser, some assist with activity programs and some help out in the office. Hope this helps.