Instead of the more common white sugar, try using demerara sugar. It has a richer, caramel-like flavor that adds complexity to the cocktail.
Replace the sugar cube with a small amount of maple syrup for a natural sweetness and a hint of maple flavor. It pairs wonderfully with bourbon or rye whiskey.
Instead of traditional aromatic bitters, experiment with cardamom bitters. They impart a warm, spicy, and slightly citrusy note to your Old Fashioned.
Substituting some or all of the whiskey with apple brandy (also known as applejack) can create a delicious variation known as the Apple Old Fashioned.
A dash of allspice dram (also called pimento dram) can add a spicy and aromatic dimension to your Old Fashioned. It pairs well with dark spirits like rum or whiskey.
While the traditional Old Fashioned often includes just an orange twist, adding a lemon twist as well can bring a more vibrant citrus aroma to the cocktail.
Before building your Old Fashioned in the glass, rinse the glass with a small amount of absinthe. It adds a subtle herbal and anise flavor to the drink.
Freeze water in an ice cube tray and add a few smoked ice cubes to your Old Fashioned. The smoky flavor can infuse your drink as the ice melts.