Nationals of the following countries may enter Thailand without a visa for a maximum stay of 90 days, no extension of stay will be granted:
Brazil
Chile
Korea (South)
Nationals of the following countries may enter Thailand without a visa for a maximum stay of 30 days:
Australia
Austria
Bahrain
Belgium
Brunei
Canada
Denmark
Finland
France
Germany
Greece
Hong Kong
Indonesia
Ireland
Israel
Italy
Japan
Kuwait
Luxembourg
Malaysia
Netherlands
New Zealand
Norway
Oman
Peru
Philippines
Portugal
Qatar
Singapore
South Africa
Spain
Sweden
Switzerland
Turkey
United Arab Emirates
United Kingdom
USA
Vietnam
Nationals of the following countries may enter Thailand without a visa for a maximum stay of 15 days, no extension of stay will be granted:
Bhutan
China
Czech Republic
Cyprus
Hungary
India
Kazakhstan
Maldives
Mauritius
Oman
Russia
Saudi Arabia
Poland
Taiwan
Ukraine
If you are only eligible for a 30 day visa, you can apply and get a 60 day visa in advance. If you overstay your visa you could be charged around 200Bht per day of your overstay. If you want to get an extention on your visa, the best way is to go across the border and come back.