I only managed to get to the island on my fifth visit. My sim had been cured of a curse and turned into a skeleton at the time, which seemed to imbue her with enormous luck on that particular exploration. I don't know whether that had anything to do with it, or whether it really was just a matter of chance. To my surprise, I also managed to get through another locked gate that I didn't even know existed. It took me to a small garden with lots of lavender growing all over. I have no idea what it's called.
I must say I was a bit disappointed with the island. Considering all the time it took to get there, I was expecting something truly spectacular, with some sort of really important treasure for my sims to grab, or maybe something really hazardous for them to deal with. But no...
It reminds me a little of the Magic Beanstalks in Sims 1 Makin' Magic. It took forever to grow a beanstalk that was climbable. And when you eventually did, and you sent your sim up the beanstalk to the land at the top, there was just this huge can of beans and a sleeping Will Wright giant that did absolutely nothing! Some players thought it was cool, but I remember feeling very let down.