depending on flight schedules sometimes you can get a stop of under 24 hours but long enough to break up the trip within the rules. I think the stopover question is best for the DL forum since the folks there would have a better handle on those sorts of rules.
I guess the responses here point out that everyone had different opinions. Personally I really like Seoul (and Singapore too). I spent a week in Seoul and found plenty to do.
Likewise Chiang Mai is a nice city. The old town inside the moat is easy to get around. It is a lot of temples though. There are some nice parks and interesting places nearby if you like nature.
I am surprised anyone would recommend Phnom Penh over SR. PP is traffic choked-nightmare