According to Archie Thomas in his book 50 Amazing Cocktails Inspired by Harry Potter, the Boy who Lived would be a classic mojito. Here's his recipe for this delightful beverage.
Harry Potter Classic Mojito
2 parts white rum (we used Bacardi)
3/4 part simple syrup
1 part lime juice
3 parts soda water
a few mint leaves
slice of lime
Add the simple syrup, lime, and mint leaves to the glass - muddle. Fill the glass 3/4 full with crushed ice. Pour in rum and fill with soda. Stir gently. Garnish with mint and lime.