New York Times ran a series on mocktails and I particularly like this one.
1/2 cucumber 4 thin slices of lime 2 sprigs mint Tonic or soda water, to taste Peychaud's bitters, to taste (optional)
Using a vegetable peeler, peel the cucumber lengthwise until you have long, wide ribbons.
Place a few slices of lime and mint leaves in the bottom of a Collins glass, or other tall glass, and, using a wooden spoon, lightly muddle them to release the juices in the lime and oils in the mint. Add a few strips of cucumber and fill the glass with ice (crushed, if you’ve got it).
Top with tonic water and a few dashes of bitters, if using.