She's 16? Then she's definitely going to enjoy going to the city. If nothing else, at least she'll enjoy shopping.
Most of the discount fares on CX don't earn FF miles on partner airlines, and many don't even earn Asia Miles. So she may get no miles at all for her fare on CX. If miles are important, you may have to look elsewhere. Like the BKK connection you mention. Bangkok is a fun place to spend 10 hours too.