I drink everything from ristretto doubles on up to Foo-Foo city.
While some syrups are gross, some are really good. These are my favorite additives:
Real Mexican Chocolate
French Vanilla, Toffee, Orange syrups
Carmel, Chocolate Sauce from Ghiradelli
My kids favorite "coffee" drink doesn't even have coffee at all. It is steamed milk, carmel sauce, mexican chocolate.