There are many kinds of citrus fruits in local markets, not just the usual oranges which are available at any time of the year, but also several smaller, more fragrant and flavourful varieties with skins that are easy to peel. They are collectively known in English as mandarin oranges, and in Cantonese as kum. The sweeter varieties are wonderful eaten out of hand, and they also make delicious freshly-squeezed juice. I like to peel and section the fruit, removing all the white threads, then freeze the pieces for about 30 minutes - it's like eating an orange popsicle. You can also use the juice to make mandarin orange curd - just use your favourite lemon curd recipe, but decrease the sugar slightly.
You might have seen fruit vendors peeling the skins off green mandarins, then stringing up the skins to dry. The fruit, which is orange-coloured, is tart and refreshing, but the main attraction is the dried peel - or kum pei. It is used to flavour stews, soups and tonics and is especially good in Chinese beef or oxtail stew simmered with a few pieces of star anise, and sweet red bean soup. Some cooks claim that the older the skin, the more flavour it has, but I've never had a chance to compare old skin and new skin. Store the dried peel in an airtight container - it keeps indefinitely.
Also in season - briefly - are small brown fruits that look like tiny apples. The mut jo grow wild in the hills of Beijing. Because the fruit is not cultivated, it's expensive at about $80 per catty. It's crunchy and sweet, with a slightly dry flesh. You can eat almost the entire fruit, except for the tiny core.