It is now possible to visit China on a 72 hour visa free basis so route via Beijing (China Airlines), Shanghai (China Eastern) or Guangzhou (China Southern) for a glimpse of China. Fares are low but the journey can be lengthy.
Malaysia and Singapore
You have to fly past Thailand to get to either of these two countries but both Kuala Lumpur (Malaysia Airlines) and Singapore (Singapore Airlines) are fascinating modern Asian cities to visit. These high quality airlines offer direct flights to several Thai airports so it is not necessary to route back via Bangkok.
State of the art and electronics obsessed Seoul (Asiana, Korean Airlines) can be visited on a lengthy route to or from Thailand for a spot of gadget shopping.