For Europe in general, nothing compares to Madrid, Barcelona, Paris and Berlin in terms of quantity, range of options and accessibility.
Athens is reasonably hard work but cheaper. However, it can't be compared to the destinations above in terms of easy access and quantify. There is a thread here about Athens that will give you a very good idea. Also, sightseeing-wise and I know it is subjective, it is not the most attractive place other than the Acropolis area and Plaka.
If you are feeling adventurous, I think Istanbul is worth trying maybe combined with one more traditional destination.