Thais don't drink water straight from the tap and nor should you; plastic bottles of drinking water (nam plao) are sold country wide, even in the smallest villages, for around 5-10 Baht. Cheap restaurants and hotels generally serve free jugs of boiled water, which should be fine to drink.
Bottled and canned brand-name soft drinks are sold all over the place, with a particularly wide range in the ubiquitous 7-11 chain stores. Soft-drink bottles are returnable, so some shops and drink stalls have an amazing system of poring the contents into a small plastic bag (fastened with an elastic band and with a straw inserted) rather than charging you the extra for taking away the bottle.
Night markets, guests houses and restaurants do a good line in freshly squeezed fruit juices such as lemon (nam manao) and orange (nam som), which often come with salt and sugar already added, particularly upcountry. The same places will usually do fruit shakes as well, blending bananas (kluay pan), papayas (malakaw pan), pineapples (sapparot pan) and others with liquid sugar or condensed milk (or yoghurt, to make lassi). Fresh coconut water (nam maprao) is another great thirst quencher- you buy the whole fruit dehusked, decapitated and chilled; Thai are also very partial to freshly squeezed sugar-cane juice (nam aoy), which is sickeningly sweet.
The larger restaurants keep their soft drinks refrigerated, but smaller cafés and shops add ice (nam khaeng) to glasses and bags. Most ice is produced commercially under hygienic conditions. For those travelling with children or just partial themselves to dairy products, UHT-preserved milk and chilled yoghurt drink are widely available (especially 7-11 stores), as are a variety of soya drinks.
Weak Chinese tea (nam chaa) make a refreshing alternative to water and often gets served in Chinese restaurants and roadside cafés. Posher restaurants keep stronger Chinese and Western-style taes (chaa) and coffee (kaafae), which is nowadays mostly the ubiquitous instant Nescafé. This is usually the coffee offered to farangs, even if freshly ground Thai-grown coffee notably several kids of hill-tribe coffee from the mountains of the north is available.
If you would like to try traditional Thai coffee, most commonly found at Chinese style cafés in the south of the country or at outdoor markets and prepared through filtering the ground through a cloth, ask for ''kaafae thung'' (literally, ''bag coffee''), normally served very bitter with sugar as well as sweetened condensed milk alongside a glass of back tea to wash it down with Fresh Western-style coffee (kaafae sot), whether filtered, espresso or percolated, is mostly limited to farang-oriented places, international-style coffee bars and big hotels, in Bangkok, Chiang Mai and beach area. Tea and coffee are normally served black, perhaps with a sachet of coffee whitener on the side.