Add cucumber slices and mint leaves to a shaker. Muddle gently.
Add limoncello, vodka or gin, lime juice, and simple syrup. Add ice and shake for 15 seconds.
Strain into a highball glass filled with fresh ice.
Top with soda water and stir gently.
Garnish with a cucumber ribbon or mint sprig if desired.
Notes
Vodka keeps it crisp, gin adds herbal notes. Adjust syrup and lime to taste. For a mocktail, use lemonade and cucumber juice. Serve ice-cold and enjoy outdoors!