You may take a public bus (or a taxi) in the morning to Omalos and walk through the famous Samaria Gorge. An alternative is to take the ferryboat (http://www.anendyk.gr)
Santa Irene Gorge
Driving up (by bus early morning or by taxi) to the village of Agia Irini and walking down through the gorge of Agia Irini is also a very nice walk. It takes circa four hours and iti is the second most famous gorge of Crete.