Here are just a few I've used at our annual luncheons:
"Who is a great volunteer? All fingers point to you!"
Nestle Butterfinger candybar
"We are NUTS about you" Marshmellow circus peanuts
"You are a great catch for our (Program name)!"
Goldfish tiny crackers by Pepperidge Farms
"You are a Cracker Jack volunteer! Thanks!
Cracker Jacks caramel corn
"You are such a SWEETART! Thanks for volunteering!"
Sweetart candies
"We would be in a crunch without our volunteers!"
Nestle Crunch bar
"This place would be a zoo with our terrific volunteers!"
Animal cracker
"When I think of great volunteers, your name keeps popping up!" Popcorn bags

You might want to buy this really helpful catalog by the name of "TREAT 'EM RIGHT!" Tasty ideas for encouraging volunteers by Susan Cutshall. It has some really great ideas.