Okay, let's try and keep things organized:
Guangzhou - Guangzhou is an enormous city, with lots of atmosphere. If you take the time to explore it, you'll find lots of interesting neighborhoods and sights. There's a lot of diversity in the city, with everything from interesting rural areas, to urban village full of Qing Dynasty atmosphere, to modern skyscrapers. Some of the city's main sights and attractions:
Shamian Island - Lots of colonial architecture. Once the only place in China where Westerners were allowed to live.
Parks - There are some interesting, large parks scattered around Guangzhou. They nearly all have some interesting history to them. They tend to vary from fairly peaceful, to noisy as anything, depending on how many "singing aunties" are around when you happen to turn up. Guangzhou's climate keeps them lovely and green.
Specific sights - An old mosque, the odd old temple, a famous clanhouse with very impressive decorations, the odd interesting church. Guangzhou's Old City is mostly destroyed, but there are some interesting older buildings scattered here and there in the place where it used to be.
Nanyue museum - One of the most interesting museums in China. The tomb of a Han Dynasty local ruler, who declared himself independent was thankfully only discovered after the Cultural Revolution. The museum is fairly well-presented, and has a treasure trove of Han Dynasty era artifacts on display.
ZhongshanLu - The city's main thoroughfare has some shophouses from the early 20th century alongside modernity, and makes for a pleasant enough walk to get to know the city.
Street markets - There are a number of interesting street markets in central Guangzhou. Most are fairly standard Chinese markets, full of noisy hawkers selling clothes, but there are other markets around where you'll find pets or exotic food items (live giant salamanders for lunch?) on display.
Wholesale - Much of Guangzhou functions as a market, selling all kinds of crap made in the factories of Guangdong to customers across China and beyond. If you're into that kind of thing, there are various malls and large shopping centers around the city specializing is the wholesale market. If you wanted to see acres and acres of nothing but red Chinese New Year's decorations, for example, there's a place in Guangzhou just for you!
The River - I think you can probably do cruises in the center of the city, but have never tried. East of the city center, there are some very interesting neighborhoods along the river, though they can be a bit tricky to get to.
Modern Guangzhou - Modern Guangzhou is more interesting (for me) than the modern districts in many other Chinese cities. There's a North-South access which is rather well planned. There's a big tower, the provincial museum (a worthy enough diversion) and if you head north enough, the city's main office district, which is full of malls and restaurants and gazillions of workers running around, seemingly at all hours. It's one of the liveliest districts of the city.
Pagodas - There are a number of pagodas scattered around Guangzhou (I think there are 4) dating from something like the 17th century. They were built to aid river navigation. They're mostly abandoned now, which make them particularly interesting to visit. One is near the Canton Tower, mentioned above.
Historic Guangzhou - There are vestiges of Guangzhou's past as a major trading center. Some lovely villages remain, in places that were once major trading centers. They were becoming popular with weekending locals when I last visited, but many of them are barely known. They tend to be east of the city. There's also a colonial cemetery hidden away, and an interesting military academy (all kinds of Chinese bigwigs, like Chiang Kai-Shek, studied there). Lots of important stuff in pre-Communist Chinese history took place in Guangzhou, and there are various places of interest around if you go looking for them.
Villages - There are some interesting villages around within the city limits, some of which have developed for tourism. See Haiou-dao, for example, a large island in the river where you can cheaply rent a bicycle and spend the day cycling through the dragonfruit fields.
African neighborhood - There used to be an African neighborhood in the city, with a wonderfully atmospheric night market. Last time I was there, the police were in the process of taking down the night market. There might still be some African restaurants around though, and presumably many of the Africans themselves are still there. Guangzhou has other expat communities, which I know less about.
Anyway. Most tourists probably content themselves with 2-3 days in Guangzhou, but if you want to explore the city in more depth, and spend more time in its markets, parks, and just wandering the streets, you could spend much longer. One thing about Guangzhou - the food is fantastic. If you like Cantonese food (and I certainly do!) then excellent, cheap meals are found on nearly every street corner. My favourites are Changfen, Chashaorou, and Cantonese-style pig feet, but there's lots of other good stuff to try.