You cannot legally work in the Kingdom of Thailand when you hold a retirement visa. Thai immigration do not make a distinction between paid employment and charity work. Therefore, you may want to think twice before volunteering to do any unpaid charity work. The only way to legally work in Thailand is to have a government issued work permit and be the holder of Thai work visa.
If you plan on travelling outside of Thailand during the one year period that your retirement extension is valid, in order to protect your extension from being cancelled, you must obtain what is called a re-entry permit before exiting the Kingdom. This does not mean that you cannot leave the country without a re-entry permit; it simply means that if you do exit Thailand without first obtaining a re-entry permit, then your retirement extension will be cancelled as soon as you leave the country. Upon re-entry, you will no longer have a valid retirement extension and will only get a 30 day visa exempt stamp.
If for whatever reason you wish to travel outside of Thailand re-entry permit can be obtained prior to leaving the country at the nearest immigration office and also at any Thai international airport. Don’t wait until the last minute to apply for a re-entry permit as the processing time for obtaining it can vary between one hour and one day depending on the location and the volume of applications being onducted at each location.
A multiple re-entry permit is more cost-effective for those who plan on making several trips out of Thailand.