Crete is certainly the biggest island in Greece. That is to say, you will need a lot of days in order to visit it all around. In other words, it consists of four prefectures; Lassithi, Heraklion, Rethymno, and Chania. Each prefecture has its own points of interest.
In short, in Lassithi, you can visit Plaka village from where you can visit and have a tour in the popular Spinalogga island arriving there by boat. It is certainly a unique experience. Moreover, Heraklion is a big enough city with beautiful squares. Above all, in Heraklion, you must visit the archaeological site of Knossos Palace.