What is the quickest way to extend a tourist visa in Bangkok from Singapore?

You would get a 30 day extension of stay based on tourism. You would go to Changwattana, Counter J.

You will need the following:

1. Your passport and:

a. a copy of your data ("picture") page

b. a copy of your most recent visa (if you have one)

c. a copy of your most recent stamp

d. a copy of your departure card

2. [MAYBE] A TM30, filed in the immigration office.

3. The forms (each immigration office may have different forms, and want to use their special forms). For Bangkok Immigration, the extension of stay request form is located in the "Files" section

4. 1x passport pictures

5. 1,900 THB.

The extension of stay will be issued on the spot, and will be added to your current "Admitted to" stamp.