Besides lemon slices, slices of orange or lime are another excellent option instead of lemon. You can even use orange/lemon peels to add more citrus fragrance to the drink.
Other garnishes: fresh or frozen cranberries, rosemary sprigs.
For simple syrup: it can be kept in an airtight container, under refrigeration, for up to 1 month.
The nutritional value hasn't included the amount of simple syrup yet.