If it's soon, then Bieszczady will be nice. Otherwise - Poland is not that good in late autumn. In winter you can go to the Tatra mountains.
In terms of cities - Warsaw, Krakow, maybe Tr-city. There are plenty of small towns with atmosphere (Lancut, Kazimierz Wielki, Zamosc, Malbork) with historical sites, but roads in Poland are not very good and it can take time to get there.
All depends what you want to do on your trip and how much time you have.