Are you trying to avoid the alcohol or just the bourbon? If the latter, I'd think rum, brandy, or cognac would be fine. If the former, I'd go with apple or orange juice. It won't taste the same but it should be okay. One online source suggested using vanilla extract, though I'd go with maybe 2 tsp. and maybe make up the difference with water if you have to. Come to think of it, most any dark soda -- Coke, Pepsi, Dr. Pepper, RC -- should also work well.