On a round-trip award, you're allowed one stopover and one open jaw. Connections less than 24 hours are just connections, not stopovers. If you have a stopover, it can be as long as you like, up to ticket validity (that is, the last segment of itinerary must start within 1 year of ticket issuance, so a stopover could easily be 11+ months if you ticket near date of outbound travel). Assuming your outbound originates in US and flies to BKK, then on your return leg you can end in any other US city and have that be your allowed open jaw.