This is the kind of drink I mostly feel like drinking if I'm out at an event. It's simple and fresh without being too sweet. You can substitute the lime juice for lemon if need be.
1/2 cup freshly squeezed lime juice (approx 2-3 limes)
1/2 cup fresh mint leaves, torn up a little
1 tbsp sweetener (sugar, maple syrup, honey, agave - optional)
Mix the first three ingredients together in a jug.
Pour a big slosh of this mixture into a large glass.
Top with ice cubes and soda water.