What documents do I need to present at the Chiang Mai immigration office to extend my 60-day tourist visa as a US citizen?

To extend your 60-day tourist visa for another 30 days in Chiang Mai as a US citizen, you will need to present your passport. Additionally, a TM30 receipt may be required, though this can vary by case. It's advisable to check directly with the Chiang Mai immigration office for specific requirements.