We visited Klein Curaçao on a day trip. It's an incredible tiny Island south of Curaçao. It takes about 1,5 to 2 hours with the boat to get there but it is worth it. On the way, you might be able to see dolphins and you can snorkel with turtles around klein Curaçao.
It should be on your "Must Do"-List when traveling to Curaçao!