These are a couple of ideas I read about that I am using:
When I signed up our workers, I got them to write down their favorite drink and snack. I am going to go around to the Sunday School classes and get people who are not VBS workers to volunteer to be "support people" for the workers. Those supporters will be expected to do things like pray for them, provide encouraging notes, help set up and take down decorations, have their favorite drink waiting for them one night, their favorite snack another.
Also, we are making little arrangements for the tables in our fellowship hall. We are putting silk greenery and a tropical flower in red take-out boxes. They will also have a pair of chopsticks sticking out of the arrangement. We will give these to the lead teachers/dept leaders at the closing ceremony.
Another way that we thank our workers is to buy each of them the VBS t-shirt.