Vietnam in October is fairly nice in the north. Starting to cool down at night a bit. Spend a week to ten days in Vietnam going from north to south (Hanoi, Halong Bay (and possibly Ninh Binh), Danang/Hoi An, and Saigon) and then then 3 days in Siem Reap, Cambodia to visit the Ankor Temple complex and then a few days in Thailand (Bangkok and the nearby Ayuthaya temple areas and maybe fly down to Phuket or take a bus to Pattaya).
@fitobugger2017@tebessa fully recommended Ayutthaya, impressive place, I can't even fathom that place during the cusp of the Siam civilization, as ruins they are impressive!!!