Bring the agave, water and two sprigs of thyme to a simmer in a saucepan. Cook and stir occasionally, until water and agave form a light syrup. Remove from heat and set aside to let thyme flavor steep; cool completely. Divide thyme syrup and unsweetened cranberry juice among 4 to 6 ice-filled glasses. Top with chilled seltzer to taste, and garnish with fresh flowering thyme.