I won't give any advice about visa regualations for US pp holders, but I guess you'll find the appropriate informaiton in another thread.
Look up what visa is necessary, take the metro (or a taxi) to downtown, enjoy Guangzhou sights (there are plenty), maybe head for a hotel (prices very reasonable) and head back to the airport 21 hours later.
CAN airport is my favorite airport to connect in China, as it is fairly small and compact. That said, it's a typical Chinese one what concerns dining options- however, I would generally not spend 23 h at any airport in the world but head out to the city...