Put a handful of California grapes (any color works, but red are particularly striking) in a pint-size bar glass. Muddle for half a minute to break down the grapes, releasing their juices and separating the skins from the flesh a bit.
Add remaining ingredients, except for soda, to the glass. Add ice and top with a bar shaker and shake vigorously.
Pour into serving glass. Top with a splash of soda water. Skewer three or four California grapes and place on top of the glass as garnish.