A museum is where you can go see things of historical significance... although "significance" is vastly different for different people. Some small towns have museums about the… history of the town, which is interesting to people who hold the town in some affection, and not so interesting to people who have never been there or who are trying to escape to a larger city. :).
There are museums that have specific themes... art museums are common, for instance. There are also Railroad museums, Tractor museums, Doll museums, Military museums, and almost anything else you can name. (MORE)
To get the museum to let you in on Counterfeit Island, you have to go through the Underground Tunnel, then exit the tunnel, where it leads you to the City Docks. There will be… a man waiting near the exit and he'll tell you to get a job at the museum, then meet him at night. You go to the museum and tell the guard you're there to apply for the job. He'll let you in the museum. (MORE)