Beijing - 4-6 days for all the cultural sites
Hong Kong - long weekend, mostly for shopping and nightlife
Sanya - few days for beach holiday
Shanghai - 4-6 days, with day trip to Nanjing, Hangzhou, Wuxi and/or Suzhou
Suzhou - see above
Tianjin - 2-3 days - temples, mosque, section of the great wall
Wuxi - see above

There are other cities i would recommend like Guilin, but may not fit your Marriott itinerary.

For public transport (in the big cities), other than during rush hour, subways and buses are fine. I personally prefer my car and taxis. Shanghai to the surrounding cities can be done by train (D trains - the fast ones). For the rest, air travel is recommended.

Feel free to contact me for any help when in Shanghai