Savory ideas: your favorite shredded cheese, feta cheese, diced ham or chicken, bell peppers, tomatoes, mushrooms, spinach, scrambled egg, Arugula salad, and maybe some dressing or sauce (vinaigrette, Italian, tzaziki, ranch, curry, Thousand Island, etc)
Sweet ideas: Nutella, cookie butter, whipped cream, bananas, strawberries, raspberries, fruit jam