First, I recommend that you start off in Chania and take a day trip to Samaria Gorge if you like nature and Elafonisi Beach. Then, make your way to Rethymno and enjoy the beaches and the culture of the city. After that, I recommend you go to Heraklion and visit Knossos especially, the ancient sites are a must see there. You don't have to hit all of these cities, but I really recommend it if you want to get a good feel of Crete.