Overland Kunming to Hong Kong
South West China is, in some ways, the most exotic area of China, largely populated by various ethnic minorities with interesting handicrafts, languages, folk music and so on. It is also a poor area, relatively cheap to travel in overall.
Some areas have been or are being developed for tourism; there you can expect to find some modern amenities, Western food, English speakers, and (by Chinese standards) high prices. Once you get out of those areas, expect none of the above.
For some parts of this trip anyone who does not speak Chinese will definitely need a Chinese phrasebook.
On the overnight busses, you have to remove your shoes on boarding and put them back on to get off for food and restroom stops. You may want to buy a pair of "kung fu" slippers (15 to 20 RMB) to make this easier if you normally travel in boots.
The two end points, Kunming and Hong Kong, both have good air links to various places. See their articles and Discount airlines in Asia for details. Both also have rail connections to the rest of China.
From Kunming, the most interesting route is to travel either by train or bus into Guizhou, a relatively poor and untouristed province with many ethnic minorities, then by bus through less travelled country to Guilin. This article treats that as the main route. Various other possibilities are under "alternate routes" below.
Kunming to Anshun
Anshun is a moderately interesting town, the hub of Western Guizhou. China's biggest waterfall is nearby and there are also a number of caves in the area.
The overnight Kunming-Anshun bus is not recommended; it is at least 16 hours and neither clean nor comfortable. Take the train or break up the journey with an intermediate stop.
Anshun to Kaili
From Anshun, it is a couple of hours by bus to Guiyang, the provincial capital. This is not a particularly interesting town, but might be worth looking around.
A possible side trip would be to go North from Guiyang to Zunyi. This is an important site for Chinese Communist history, scene of the Zunyi Conference during the Long March.
From Guiyang, it is about four hours by bus to Kaili, a trading town with many interesting villages nearby. This is the hub of Eastern Guizhou, probably the most interesting place on this route.
Alternately, you could take a train to Kaili from Guiyang or even directly from Kunming. Trying to get the train from Anshun is not recommended; it is hard to get seats except at major stations.
Kaili to Guilin
From Kaili to Guilin is a fairly long journey on a series of busses. There are some overnight busses, but the scenery — mountains, terraced fields, ethnic vilages — makes it worth travelling by day.
Sanjiang is worth a stop. It is the capital of a Dong minority area and has some sites and shopping.
Longsheng, between Sanjiang and Guilin, has the famously scenic "Dragon's Backbone" rice terraces. If you haven't already seen more than enough terraced fields in Guizhou, you might stop for these.
For most Western tourists, the main reason for going to Guilin is to get to Yangshuo, an hour and a bit South by bus. However, Guilin itself might be worth a stop. Certainly it is very popular with Chinese tour groups.
Guilin to Hong Kong
From Guilin, continue to the tourist town Yangshuo, then:
Either way, you can reach Hong Kong.
Kunming to Nanning or Guilin by rail
From Kunming, you can take a train through sensational mountain scenery to Nanning, the capital of Guangxi. From Nanning, it is a short trip by bus or train to tourist center Guilin. See above for routes from Guilin to Hong Kong.
There are also flights possible to both Nanning and Guilin from most major cities in China.
Nanning to Hong Kong along the coast
Staying within China, one could also swing South from Nanning, toward the coast instead of toward Guilin, making for Behai, optionally by ferry to Hainan, then on to Hong Kong via Zhanjiang and Zhuhai.
Kunming to Hong Kong by (mostly) air
The fastest reasonably priced way from Kunming to Hong Kong or vice versa is by a plane/ferry combination, about 4 hours / 850+ RMB. You could fly direct to Hong Kong, but it is usually significantly cheaper to land in Shenzhen and take a ferry.
Many foreigners in China need to get a new visa every 6 or 12 months and Hong Kong or Macau are good places for this. For most passports you can get a multiple entry business visa with just a photo and money. The plane/ferry combination is the usual method for such "visa runs".
Kunming - Hong Kong
That's it. For about 850 RMB you can go to Hong Kong.
A very convenient way to book the ferry and plane ticket together is at the China Travel Service (HK). It is easy to find as it is placed directly in the opposite of the Chunking Mansions in Hong Kong Kowloon where many backpackers stay. Turbojet Ferry Schedule
These areas are generally quite poor and petty theft is common. Take precautions against pickpockets and watch for things like razor attacks on luggage during bus or train rides.
Armed robbery is rare, but has been reported. Victims are generally lone travellers, often female. Try to stay in a group, especially on treks through isolated areas.
Altitude sickness is a risk in some areas. especially if you go west of Kunming up toward the Himalayas.
Both cities are also well connected to the Chinese rail network. From Kunming, the possibilities are: