Teachers.net has six poems for thanking volunteers on their homepage; "The Volunteer" is short and sweet. There's an acrostic poem, some rhyming poems, and non-rhyming ones to choose from.
"Volunteer/Parent Acrostic Poem" would be good for a more sentimental person, and "What are Volunteers" may work better for a more light-hearted, humorous tone.
When it comes to thanking volunteers with a poem, less is often more. The ideal thank you poem would probably be one that the volunteer could read in a few seconds and then look up from the card and express gratitude. A long poem is often too much for a thank you celebration.