Save for the grubby looks I get when I admit it - Guangzhou is one of my favorite cities in China. The food is good, the weather always fine (except when it's chucking it down) and the culture emerging. Maybe it's just the super-smooth roads but explore a random hutong in Haizhu, particulary the area north and south of Jiangnan West Road and perhaps you'll get a feel for what I mean.
Getting there from Hong Kong. Don't book in advance, then miss your train and have to buy a new one. Just buy it when you get there - the train is massive.
Getting there from Shanghai. Wait until Q1 2011 when the world's fastest train (or high-speed depending who's commenting on this) will arrive 15 minutes before you left.