After an individual Mii has reached level 20, you'll unlock the option to give them any hats or clothing already in your collection. This is feature is available only for awarding Miis when leveling up.
Every Child needs a Album! So when they grow up, the Parents give their child's album to you.
After a couple has married and produced a child, you'll unlock an option to take children's and family portraits in the Photo Studio.
When you first unlock the Concert Hall, you'll only be able to do Solo Performances. To create Group Performances, two or more Miis have to learn the same song style.
By completing certain tasks, you can unlock additional areas around the island.