Expenses here follow the weather, with cheaper airfares in the rainy season everywhere (see below for details). You can get cheap fares to China, Korea and Japan during our winter months, often as low as $899 for the plane and five nights in a hotel, sometimes even less. Generally speaking, the high season in those three countries is summer, while the high season in the sub tropical nations further south is more or less during our winter. Avoid Golden Week (late April through early May) in Japan at all costs, as everyone travels domestically then. In Central Asia, our summer is the best time to visit, and in West Asia, the same.
East Asia--China is vast, its weather pattern extreme, ranging from sub tropical in the southeast (near Hong Kong and Macao) and southwest (Kunming, gateway to "Shangri-La") to frigid (in the Himalayas and Tibet, not to mention Manchuria). Only the southwest has a monsoon-influenced climate, hot and wet. The best time to visit almost anywhere in China is between late September and early November. From late March through early May is second best, but quite nice in Beijing, Shanghai or along the Yangtze River. By all means, avoid summer (actually mid May through late August), the rainy season for most of the country. During winter, forget about visiting Tibet, Manchuria or most of the Silk Road spots in the northwest--it's too cold and many facilities just close down.
Hong Kong and Macao are best, in this semi-tropical setting, between September and March. Avoid April through June, the rainy season, and July and August, the hottest months.
The best time to see Japan is during the cherry- and plum-blossoming time (March through May) or when the fall foliage colors attract (September through November). Avoid June, when the monsoons bring down buckets of rain. July and August are hot, but full of festivals. Korean weather is similar to that of Japan's, but colder in winter.
Taiwan, also sub-tropical, has summery weather from May through December, though the best months are March and April, November and December. The mild winter is just two months long, December and January.
Southeast Asia--Cambodia, Myanmar (Burma) and Thailand are hot all year round, and fairly steamy. Winter is best (November through February). June through October is the monsoon season to avoid. The average temperature in these parts is about 80°F.
Indonesia bears up under rain throughout much of the year, and it is always humid and hot. If one month can be better than the others, it is probably February.
Malaysia and Singapore have a different rainy season (October through January), from their neighbors to the north. The temperature here remains about 75°F throughout the year, give or take a few degrees in either direction.
Philippines has a short winter, with November being the best month of all to visit. Otherwise, count on hot and humid.
Vietnam. Avoid the rainy season from March through October. Otherwise, the hot weather of the dry season is to be preferred.
South Asia--In India, the tourist season lasts from mid October through March, when the weather is at its best--dry and moderate. This does not include the southeast, where the monsoon season is from November through January. (The southwestern monsoon season is from May to September.) If the fighting in Kashmir has stopped (who knows when?), the best time to go there (and to the Himalayas to the east) is from April through October.
Pakistan (again, when the current unstable situation dies down) is best between October and March, when its coastal areas are like the Mediterranean, though you should avoid the mountain regions in winter.
Central Asia--The bad news: The weather in the "stan" countries (Afghanistan, Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan, Uzbekistan,) is extreme, ranging from freezing or below in winter to the 80s and 90s in the summer. The good news: It rains very little in this area, mostly in March and April and again in October and November, as the seasons change. In the mountains, it always seems to be cold, but in low-lying areas (where most tourist destinations are), the best months are May and June, then again in September. Summer can be dangerously hot, especially in the deserts and on the arid plateaus.
You can visit Iran now if you dare, the best time of year being from mid May through the end of October.
West Asia--In Israel, Jordan and Syria, the tourist season is from mid April through mid October, though summers (July and August in particular) can be terribly hot. Don't even think about spending time in the desert areas without adequate transportation, water and protection against the sun. But November through March can be pleasant at Israeli and Jordanian resorts on the Red Sea, temperatures ranging from 60°F to 80°F.
Asia is so vast (being the largest continent) and so diverse (Shinto in the east, Judaism in the west, Islam in the center and Hinduism in the south), that summarizing activities here can only hint at the remarkable opportunities available. You can traverse the Silk Road, or most of it, coming close to the fighting in Afghanistan, Pakistan and other places as you trudge forward. You can lie on the beaches of Thailand or Goa, visit holy men in India, climb a sacred mountain in either Japan or China, dive in clear waters in the Philippines (stay away from Mindanao, though!), or learn temple dancing in Bali, just to mention a few examples. There is boating in Japan and China, rafting and hiking in Nepal, mountain climbing in Nepal and Bhutan, and luxury railroading between Bangkok and Singapore. Indoors, you can visit the great temples of Japan, Korea and China, the palaces in the same countries, and the fine museums, especially those of Tokyo, Kyoto, Shanghai and Beijing. Don't miss the archeological wonders of Angkor Wat or Borobudur if you can manage the trip, or, second-best, the wonders of Pagan or Ayuddhya. If you want to traverse the classic Yangtze River, do so soon, as the Three Gorges Dam (the biggest on earth) will lead to filling up the area starting next year, and will continue to do so until 2010, when the gorges will have been changed, some disappearing forever beneath the risen water. See Shanghai, where the new Pudong District's skyscrapers make some New Yorkers feel like country bumpkins. And don't forget Hong Kong, which still has the world's most beautiful harbor.