1 1/2 cups water
1 1/2 cups agave nectar
1 1/2 cups dried lavender blossoms
1/4 cup mint leaves
2 1/2 cups pineapple juice
2 cups club soda, or to taste
8 slices lime
8 sprigs mint
Bring water to a boil in a small saucepan. Reduce heat to low; whisk in agave nectar until dissolved. Remove from heat.
Stir lavender blossoms into the saucepan. Let syrup steep, about 15 minutes. Strain into a bowl through a strainer lined with cheesecloth.
Refrigerate syrup until chilled, at least 2 hours.
Pour 1 1/2 cup syrup into a glass measuring cup. Add mint leaves; mash gently with a cocktail muddler. Mix in pineapple juice.
Fill a pitcher with ice. Pour in pineapple juice mixture. Top off with club soda and stir. Serve garnished with a slice of lime and sprig of mint.