One thing we used to do (but have since stopped) is recognizing two volunteers in the monthly newsletter. One was nominated by the teachers and one was nominated by the PTA board. A short description of the things they helped with was listed, and they each won a lunch in the school cafeteria!
We have one big award at the end of the year -- usually for a volunteer who has done a lot throughout the years and is in the last year at the school. It's given at the awards ceremony. They receive a certificate and a charm in the shape of the school's mascot. Other volunteers are also recognized with a certificate.
There was actually a really good write up on a "Gem" Event (Go the Extra Mile) on this site. We used it to award our volunteers and each volunteer just got a different kind of certificate whether it was Gold, Silver, Diamond or just recognition based on the estimated amount of hours they volunteered. We are actually going to try to keep really good track this year and maybe hold an event like the one they mentioned in the article. If you just do a search on the word Gem you should find it.
We are trying to develop a volunteer recognition program, and one of the first questions was how to differentiate people who only volunteer once, and those that volunteer often?
Do you give everyone a certificate? Or are there different "levels" of certificates?