Coconut water (น้ำมะพร้าว naam ma-phrao), iced and drunk directly from a fresh coconut is a cheap and healthy way to cool the body. Available at restaurants and also from fruit juice vendors.
Fruit juices, freezes and milkshakes of all kinds are very popular with Thais and visitors alike. Most cafés and restaurants charge 20-40 baht, but a bottle of freshly squeezed Thai sweet orange juice (น้ำส้ม naam som) - which really is orange in colour! - can be sold on the street for 15-30 baht. Thais often add salt to their fruit juices-- an acquired taste that you might just learn to like. Thais also like to have basil seeds in their iced fruit juice sold on the road. They look like small jelly balls down in the bottle.