Plan on visiting the Darwin Institutes facility.
It is there that you will see the difference between the tortoises from the various islands in the archipelago, and find out how they keep the various species segregated and "grow" them for 5 years prior to returning them their native islands. And this is where you see Lonesome George, the sole survivor of the tortoises from Pinta Island. He's lonesome because he chases everyone away, whether male or female. You'll learn how the Institute is attempting to get him to reproduce, including an attempt at cloning. The same is being done for the iguana.
You will have the opportunity to get up close and personal with the giant tortoises (just don't touch them).