If you are into lemon tea, then you might want to try the Vita Lemon Tea. Its one of the most popular drinks in HK.
Its nice because it has such a strong tangy lemon flavor.
There are a few versions, like low sugar, the original below, the honey lemon, lemon lime flavor or ceylon tea flavor. You can get this anywhere, from restaurants to Circle K to 7-11 to supermarkets.
Some of my colleagues really like the cream soda, I know they have zero cream soda version (if you are lucky, I don’t see it often)