What should I do about my expiring Thai visa before my flight?

You might get an overstay stamp due to office being closed for holidays .( Aren't they closed 29th December -2nd January) ?

Planing

If you want to not pay for overstay you must get out of Thailand on the 30.th! If you don’t you must pay 1500 baht in immigration in the airport when you leave Thailand. You can save 500 baht if you go through immigration before midnight!

You will pay 500tbh at Samui for each day you have overstayed and get a ‘naughty’ stamp in your passport. You will also need to lay low in Samui in order to prevent encountering any officials. It may affect future visits, but all depends on the IO that you get on the way back into Thailand.

The 7 days to leave Thailand stamp is an option, but again I’m not sure if this is detrimental in the future. Leaving the country and coming back in on a 30 day exemption will be costly, but that’s your only option if you don’t want to take risks for future visits I suppose.

You only get 1 30 day extension on Visa exempt and 1 on 60 day tourist visa.

If you have the 60 day visa then go to immigration and extend it.

If you don't have a visa and you extended your 30 days visa exempt entry another 30 already you will go into overstay. Can pay on departure but don't get caught by police before you get to airport as they can arrest you.

Can try apply for another extension and hopefully get the extension declined 7 day stamp to leave