Usually, places that are good for a long layover:
– Are culturally interesting or have at least 1 very interesting site.
– Make it easy and quick traveling between the airport and the city/site.
– Have no/easy visa requirements/fees (I’m American).
1/ Hong Kong
As long as you can enter the city (a few countries are persona non grata in HK, and require a visa even for short visits). The airport is very nice for travel (that’s my home base, I use it every week), but not so much for stopovers. Shopping there is horrendously expensive. I had to stay 7 hours in HK between 2 flights, a couple of times, and I was dying of boredom.
It takes all of 20 mn to go downtown. Great city to visit – especially if you’re a first-timer. The airport has indeed lots of entertainment, food, and relaxation spots – but it is still an airport, and one with miles of corridors to walk. Duty Free shopping is, like Hong Kong, suicidal for your wallet. There are very few airports where DF shops are actually cheaper than downtown — Incheon airport and Korean airlines come to mind.
3/ Beijing, Shanghai, Guangzhou, Shenzhen etc
There’s a newish scheme in place that enables passengers with an onward ticket (but not a return ticket to the departure city though) to enter and stay up to 72 or 144 hours in China without a visa. Go ahead and enjoy.
Not much to see in the city per se, and travel to/from the airport is costly, time-wise (and money-wise if you’re foolish enough to take a taxi). But again for a first-timer there’s enough to fill a day. And the duty free shops are some of the cheapest in the world (a large part of my business is spent selling to DF operators. You’d be surprised how much money these people make…).
Setting aside the issue of cost – everything in Japan is expensive, including public transportation – there’s no reason to stay in Narita 18 to 24 hours (let alone Haneda, which is 20 mn from downtown). The airports are average at best, whereas the city has a lot to offer.
The airport is a bit of a mess but there’s food and shopping and even a day hotel in the airport – so if you’re reluctant to go for an adventure you can stay inside and survive. But… You’d be better off going out and visit. The main/only deterrent to visit BKK under a few days is the time spent/wasted in transportation. Horrible gridlocks, mediocre and sparse public transportation. There’s an airport link that’s fast, cheap and convenient – get off at Phaya Thai, and change for the BTS line, where you can go about anywhere you’d be reasonably expected to go to as a tourist.
7/ Kuala Lumpur
Again you’ll spend untold amounts of time in traffic. There isn’t that much to see in KL either — there are much better places to visit in Malaysia. The airport isn’t exactly a place I’d like to roam about 18 to 24 hours either.
A hellhole best left alone. Both the airport and the city.
Horrible gridlocks. Wonderful food but not much to see. Shopping malls, shopping malls, shopping malls, shopping malls… Did I mention the traffic was horrendous?
Nice city with a few things to see, excellent food and a potentially useful downtown airport (Songshan) if you’re on your way to Japan, Korea, China, as this renovated airport (the former international airport, demoted to regional when Taoyuan was built, and, like Haneda in Tokyo and Gimpo in Seoul, put back again to use for international flights due to congestion. Very useful as it’s downtown, and goes to airports like Haneda, Gimpo, or Hongqiao in Shanghai).
The newly opened train to Taoyuan airport is cheap, fast and convenient. No reason to take taxis anymore.
Technically not in Asia but a big transit hub between Europe and Asia. Unless you’ve business there, the probability of going there on purpose is very low. Might as well grab the opportunity.
I doubt places like Vientiane, Phnom Penh, Saigon, Hanoi, Yangon are much of airlines hubs. They all require visas too.