Crete is full of caves. On 2017 we visited IdeonCave and SfentoniCave and on 2019 MatalaBeachCaves. If you have time to see only one but would like to see something great I recommend to see MelidoniCave. I like its lightning because it is more natural than SfentoniCave. The entrance is open so the air feels better and easier to breath (SfentoniCave has closed entrance). The cave is also much higher than SfentoniCave. Unfortunately you need a good ability to walk (also stairs) when stepping down into MelidoniCave; but it is much easier to access than IdeonCave.
Can you spot a huge man on the pic? There is his head right above the center of the pic. He has a huge body and he seems to sit and think.