Hong Kong Heritage Museum
The museum is open 10:00-18:00 (til 19:00 on Saturday and Sunday), closed on Tuesdays. It's located across the river (use the pedestrian footbridge) from Che Kung Temple Station, Exit A.
Admission is HK$10, free on Wednesdays. There is a museum pass including several other museums for $100 for one year, $50 for six months, and $30 for a week. If you can't visit during a free time, the pass may be a good option. The six month and one year passes also offer discounts, including a substantial discount at Hong Kong Disneyland.