Have a look here for suggested iternaries
http://www.trailfinders.com/index.htm
Its quite expensive leaving Hong Kong - you could fly from Shenzhen instead as an alternative. You get the ferry from Kowloon to Shenzhen and then jump on the free shuttle bus to the airport.
http://www.chinatravel1.com/english/plane/sz8-book.htm
But you will need a Chinese visa to do this.
Ooooohhhh so many ideas. How about Beijing - Xian - Shanghai ? How about Hanoi - Ho Chi Minh (travelling down by train/bus) ? Flying into Vietnam is quite pricey though. How about Bangkok - Chiang Mai? How about Kota Kinabula - Orangutans ?
Remember to check the weather situation in each place though - it can be lovely in one place and chucking it down with rain in another.