From Ancol, you can take a boat to the many islands in Pulau Seribu (can be literally translated to A Thousand Islands). It's a perfect getaway from the hustle bustle of the city.
Be careful with 'guided visit' to the watchtower. Even if you refuse, they may follow you around telling stories about the place. They'll ask for more than you would ever pay as entrance fee.
I'm not sure which is the most impressive item, but the solid jade four poster bed is certainly in the running, as is the thousand piece porcelain dinner service given by Queen Elizabeth.
