Re-posting this due to moderator disabling comments before I could receive a helpful response:
Does anyone know if it's possible to fly to Luang Prabang and come right back to Chiang Mai the same day on a later flight? This is to get a visa exempt stamp when arriving back in Thailand obviously. Thanks.
TLDR : Answer Summary
It is possible to fly from Chiang Mai to Luang Prabang and return on the same day to obtain a visa exempt stamp, as there is no required duration for being outside of Thailand for this purpose. However, travelers should note that there is only one flight available each day, necessitating an overnight stay in Luang Prabang. Additionally, although there are no strict limits on acquiring multiple 30-day visa exempt entries by air, immigration officials may request proof of sufficient funds, onward travel, and accommodation when re-entering Thailand.