Cointreau and Aperol both have orange flavors but they aren't super similar. Aperol is much more bitter and has other tart flavors in it. Cointreau, on the other hand, is pretty straightforward with the orange flavor and accents a drink well with a hit of citrus. You could use Aperol in this recipe since it is slightly similar to an Aperol Spritz, but the flavor would most likely overpower the cucumber and thyme.