What should I see/do between Beijing and Singapore?

I fly out (from London) to Hong Kong in early July, make my way along the coast up to Beijing, then back down through the hinterland to Hanoi, Saigon, across Cambodia to Bangkok, down the Thai peninsular to Kuala Lumpur, over the bridge to Singapore, and fly back from Singapore in early September.

This map gives a rough idea of the itinerary. I will be spending about 4 weeks in China & 4 weeks in SE Asia.

Please advise on anything you recommend doing in or between those places marked on the map.

General advice on travelling this journey alone overland is also very welcome.