I didn't like HK if I'm honest. Too hot, busy and expensive for my tastes.
Not been too many places in the south. Most of my trips to China were in the North. But I did go to Chengdu, that's where the Pandas are, so worth a visit. The street food vendors there can be amazing too, some really lovely food (it is in Sichuan province). Although it is a little out the way for the route to Shanghai.
Xi'an would be another good visit, go see the Terracotta Army.
They are the two places I would definitely go see again if I had the chance.
Never been to Shanghai, but spent a LOT of time in Beijing. Love that city. Although I did feel I couldn't stay there for any length of time, because I just drank too much, lol. Obviously there is Tiananmen to go see, and the forbidden city is right there too. And the Birds Nest Stadium is worth a visit.
I used to stay up in the Lido district when I was there, and was a relatively short walk to some Art district (can't remember the name), but it was a great place to spend a day, with lots of exhibitions by local artists, of both proper art, and showing off some of their travells (there was a fascinating one of a woman who toured Afghanistan back in '07-08).
Food is hit and miss. Mostly it is absolutely lovely, but I did spend time on the rigs out there, where it was a REAL miss. We went to one of the best duck restaurants in Beijing (it was a business meal so I can't remember where exactly, or even the name), but it was THE single most delicious meal I have EVER had in China. It was really fantastic, and well worth hitting one of the top duck restaurants in the city.
Another favourite was their version of Hot Pot. They basically have a massive bowl of boiling soup in the middle of the table, and you order up whatever (usually shredded) meats, fish, veg you want, and boil it as you go. Usually only takes a few seconds to cook in the "soup" and you just do it with chopsticks. The soup is usually split in two, one side spicy, the other not. And you make up your own variation of a dip with a usually large selection of spices / herbs / sauces (peanut base always works for me). It is lovely, but I usually found it better done in the North.
If it's winter, and your brave, and you go further North, Harbin city is a lovely place too. They do a big park filled with ice sculptures, all lit up lovely. It really is a fascinating place. And usually have a massive ice slide you can go down. It's ace.
As with everywhere in China though, make sure you take your own toilet roll. EVERYWHERE. Out on the peeve, out for a meal, out shopping etc... It is very rarely supplied, so don't get caught out.