Do volunteers work your library? A great Idea would be to purchase some books and give them to the Library with a book inscribtion to honor your volunteer. It shares the love of the school as well as a thoughful way to build a library.
I think it would be nice to put a photo of the volunteer with a vase of flowers next to it and a small journal book. Ask the staff and/or students to write a favorite memory of that person in the book. Then, you could display it for a month or so, and then present it to her family. I'm sure they would like that very much. Patty