Here are some ideas.
• Fold 1 tablespoon freshly chopped summer savory or marjoram and 3 tablespoons finely grated Parmesan cheese into batter when adding butter (from "American Home Cooking" by Cheryl and Bill Jamison).
• Fold 1/4 cup crumbled blue cheese at room temperature into batter when adding butter (from "Come One, Come All/ Easy Entertaining With Seasonal Menus" by Lee Svitak Dean).
• Mash 2 garlic cloves with a pinch of salt into a paste and whisk into batter when adding flour. Pour batter into popover cups in thirds, alternating with a sprinkle of finely grated Parmesan cheese after the first and second pours (from "The Gourmet Cookbook" edited by Ruth Reichl).
• Fold 2 tablespoons freshly chopped chives into batter when adding butter ("The New Basics Cookbook" by Julee Rosso and Sheila Lukins).
• Bake popovers in a mini-popover pan. Remove from oven, remove popovers from pan, brush hot popovers with melted butter, then roll popovers in a mixture of 1 teaspoon cinnamon and 3/4 cup sugar (from the New York Times Magazine).
• In a medium bowl, using an electric mixer on medium speed, beat 1/4 cup (1/2 stick) unsalted butter at room temperature until butter is light and fluffy. Add 1 teaspoon honey and beat until well-combined. Transfer to a bowl and serve with warm popovers (from "The Marshall Field's Cookbook" by Steve Siegelman).
• In a medium bowl, using an electric mixer on medium speed, beat 1/2 cup unsalted butter (1 stick) until light and fluffy. Add 3 tablespoons good-quality strawberry preserves and beat until well-combined. Transfer to a bowl and serve with warm popovers (from "The Neiman Marcus Cookbook" by Kevin Garvin).