DC's signature drink, the Rickey, is "an airconditioner in a glass" as Columbia Room's Derek Brown puts it, and fiendishly simple. While the original uses bourbon, I prefer the gin version.

In a highball with ice, add in:
The juice of a key lime (and both shells, or the juice of 1/2 of a normal lime and a dash of simple syrup or an orange syrup)
2-3 oz of gin - and, for DC's own drink, use Green Hat, DC's own gin.
fill the glass with sparkling mineral water (pellegrino works well)