Where can I find the keys to the Inn rooms?

You'll come across them here and there on the island.

One is up at the RUINS.

You will see it on the grass on the left of the steps as you make your way back down to the Town Square.

One is in the POST OFFICE.

You will find it when you topple the mug on the Postmaster General's desk.

One is in the INN itself.

You will find it in the closet behind the lobby desk, hanging over the top shelf.