The Guangdong Museum is, first and foremost, a gorgeous building. One of the many unusual structures that lines the new Zhujiang New Town Plaza, the Guangdong Musuem is an easy one to enter on your own time and own terms. Browse exhibits on natural resources in Guangdong, a beautiful display of Chinese medicinal plants (though English interpretive signs were missing in this exhibit when I visited), and impressive traveling shows that you might find at the Smithsonian, the Field Museum, or other similar venues around the globe. Admission is free with your ID.
Pro tip: Couple your visit here with a stroll along the plaza and pop your head into the Guangzhou Library, next door, while you're in the neighborhood. A mid-plaza fountain show happens periodically throughout the day, accompanied by soaring music (and, at night, as with everything here, colorful LED lighting).