...what is the most appropriate way to show my gratitude?

take him out for a decent meal i guess, spend the amount of money depending on how big the favour

The way to a Chinese persons heart is through their stomach.

Pizza Hut! Really. Pizza Hut.

If that chinese person has no taste Pizza Hut does sound like a good choice, and you take take them to Starbucks afterward..

A carton of good quality Yunnan cigarettes such as 红塔 or a bottle of good quality Guizhou baijiu 白酒 such as 茅台 are also options.

Quality baijiu is gonna cost 1200+ rmb

Pizza hut is great for younger local. It really is something they could/would not normally afford. And if you are not a huge fan of local food, it is a treat for you as well.

OK I know PH is not 5 Star cuisine, but after living here a few years I find I eat much more fast food in China that I would ever eat back home.