Gulangyu, the small island with a colonial atmosphere, is the main sight, but I gather that in recent years it's become something of a hassle to reach it because they've gummed up the ferry system for visitors. You'll have to research the details.

As to money, do not sweat it. Take a few hundred RMB from an ATM -- maybe you'll have a nice meal and a taxi ride -- then exchange the excess when you reach Hong Kong. Money changers in Hong Kong offer very low spreads on RMB/HKD exchange, so the cost of getting more RMB than you need will be minimal.