@Diana ******
If you plan on being here less than 60 days, just a valid passport (6 months remaining) and a scheduled return flight. If you show up as a tourist and start bouncing across the border to reset your 60 days, yes, you will be met with resistance with immigration. That is who the Thai government is targeting, those taking advantage of a loophole. They claim that if you are doing this, you need to search and find a visa that matches your intentions in Thailand. My Visa is good for 90 days with the option to extend another 60 and extend another year after that. There is more fine detail along the way but that is the jist of things.