Greece is in the EC, so there is no such thing as Duty Free, (assuming you are holder of an EC passport). Booze and fags will be cheaper there than here, assuming you use the local stuff.
Dont know what the weather will be like, but the prices for cigs and booze at the airport is no different than at the supermarkets so you may as well get them during your stay rather than a mad rush at the end. We went in august the prices are B@H 40 euros L@B 35 euros and many other brands starting as little as 20 euros, booze was cheap also but we didn't get any of that so cannot tell you the prices for those.
Just in case you didn't know (which i'm sure you did), as in the eu there is no duty free so there is no limit to how many cigs you can buy although any more than 3200 may get the attention of the customs.