Mermaid Lagoon has, I believe, two rides outside and three rides inside. In addition, the interior houses a playground, a restaurant, a shop, and an enormous theater.
The interior portion is ALL underground and it's enormous. However, it's so well themed that you never have the feeling of being in a warehouse.
Three of the rides have already appeared at Disney's California adventure: Jumping Jellyfish, Francis's Ladybug Boogie (or whatever the mini-teacup is called), and Flik's Flyers. I have not been on these rides in Tokyo DisneySea, but in DCA my daughter, age 4, loved all three and we rode them repeatedly.
While the rides are not appealing to adults without kids, adults who take in the show, restaurant, and shop will kill a good two hours.
It's a great place and would make a terrific addition to the Magic Kingdom. Those without children are generally less generous about these types of additions.