I went to lunch a few days before I left in Kowloon City (the home of the now demolished infamous Kowloon Walled City, featured in movies ranging from the Van Damm vehicle, Blood Sport to Jackie Chan's Crime Story). This former industrial area is now a cosy residential district that has a number of Thai restaurants and grocery stores. The Thai fruit and vegetables in the small grocery stores often can't be found in regular markets elsewhere in Hong Kong so I took the opportunity to pick up some fruit for experimenting.
Tamarind and galangal:
Tamarind fruit is a sort of legume, with a hard shell with 6-12 pods. The flesh is juicy and quite sour but as the fruit matures the flesh becomes sweeter. Its grown in areas around the tropics from south Asia to Mexico and can be found in everything from agua frescas to jams.
Galangal is a root, related to ginger but tastes very different, originating in Indonesia, and is used commonly in S.E Asian cooking.
I bought a small bag of tamarind pulp (the exterior pod had been removed) and soaked it in sugar water over night. I then poured it through a fine filter to get the sweetened tamarind juice.
Grated galangal was added to the tamarind juice, then additional water added (tamarind packs a serious kick so it needs to be diluted) before carbonating in my trusty ISI soda siphon. 
(yet another thirst-quenching drink that my sister is enjoying!)
Green mango and chili
I also picked up a couple of green, unripe mangoes - tart, crispy and refreshing, they're common in Thai salads, very different from the sweet, succulent flesh of ripe yellow mangoes.
I diced a small Thai chili, cooked it in sugar water solution for about 15 minutes, then added it to the juice of two green mangos. One chili pepper gave it just the right amount of a kick in the back of your throat.
