This is a million dollar question !!

Crete is a year-round destination, is big enough that it could easily be made into a trip of is own (driving from one end to the other would take approximately 6 hours without stopping). 

With its remarkable hospitality, exceptional cuisine drawn from the island’s own local produce, traditional villages and monasteries, and pristine beaches, Crete could easily be explored for a week or more. In less than a week, however, you can still check off many of its main sites, including the bustling capital Heraklion and the Venetian cities of Chania and Rethymno, charming coastal towns like Agios Nikolaus and Sitia, the Minoan ruins of Knossos, and the striking Samaria Gorge.

A rental car will allow you to make the most of the island while maximizing flexibility and keeping costs down. However you must reserve a guided tour to visit  some of the highlights of Crete. Consider having two home bases—one each on either end of the island—in order to minimize travel time among various sites, or spend several days covering the island from one end to the other. Keep in mind, there are airports close to both the capital of Heraklion and Chania.