UA Let's you fly 2 stopovers and open jaw in SE Asia for a little over cost of RT.

So you could fly US-HKG, SIN-HKG-US as two one way (open jaw) with AA. Then fly HKG-TPE-BKK-KUL should price at like 27k UA. Add a train from KUL-SIN and you can get get 5 destinations pretty easily. Another option is a long close to 24 hour layover in SIN, yhe city is pretty small.

For locations like SGN, PHN, visa will probably be needed and 3-4 days is not enough