I know simple syrups are usually water and sugar. Sugar is dissolved in boiling water. When cooled you can add liquor to flavor. Here is one I found on the internet after I did a search for "simple syrups for cakes"
BRANDY SIMPLE SYRUP
2 cups water
1 cup white sugar
1/2 teaspoon grated lemon zest
1/4 cup brandy
1.In a saucepan, combine water, sugar and lemon zest. Bring mixture to a boil and continue boiling for 5 minutes. Strain into a sealable container, removing the lemon zest. Allow to cool, then cover and store in refrigerator.
2.When ready to use, mix in brandy. Brush onto cake layers before frosting