I ended up getting them gift cards to a local ice cream shop and a scratch ticket. I was orginally going with a "sweets for a sweet volunteer" theme but decided that was lame... so I left off the cheesy title.
I also made four different thank you cards to go with the gifts:
(this is the front of the first one)
(and the inside)
It was a very small token of my gratitude, but it seemed to go over well!
Sorry for the really bad photos. I'm having bad camera mojo today.