Here is my understanding, and other posters can correct me if I am wrong.
You need the Magic Your Way Plus ticket for admission to Pleasure Island. I guess if you only wanted to go once, it might be more economical to purchase a one day admission to Pleasure Island. The Plus ticket admits you to Blizzard Beach, Typhoon Lagoon and Disney Quest as well.
You can walk around Pleasure Island without buying admission, but you will not be admitted to any of the clubs. I'm not sure about the restaurants.