Interesting Thai food and drinks you should try. If you'd rather not, check out regular Thai Street Food.
Fish Egg Soup (แก้งส้มไข่ปลาเรียวเซียว)
Soup with yellow balls, which I found to be fish eggs. They are huge (well, at least for fish egg), one centimetre at least. The eggs have a floury taste and are a little tough, the soup is delicious but spicy.