Always check the weather

Just because it's the Mediterranean far south, this doesn't mean that the weather conditions are always going to be perfect. Crete often gets extreme weather conditions, such as big storms, heavy snowfalls on the mountains, high winds and -of course- blistering heat waves.
Especially if you're traveling by ferry and are easily seasick, high winds and rough seas can be a bit of a problem, since the journey takes 8 to 10 hours. It's an overnight journey (from 8pm to 6am or so), so for your comfort I would suggest choosing a cabin with a nice comfortable bed.

If you're on the island and about to venture on a super exciting road trip, always check the weather no matter what the season is. The conditions can change depending on the altitude, the relief, etc. For example, it's often cloudy and foggy up on high altitudes, even in the midst of the summer. Elafonissi lagoon is almost always windy (which makes it a great windsurf spot). Canyons also tend to be windy because the wind coming from the mountains gets suppresed.
Also, the summer heat is no game. Many visitors often underestimate the scorching Cretan sun. It's probably not a great idea to take long hikes in the summer heat, or if you do, always choose to hike very early in the morning. Dusk time isn't a good time either, since it will eventually get dark.