|There are two border posts close to Ko Chang. Just a few kilometres south of Trat is Hat Let, where you can extend your visa by 30 days. All you need do is to make the crossing into Cambodia at Cham Yeam. The whole process should take you no longer than 30 minutes.
First, get your Thai exit stamp, walk into Cambodia, then go to the office and fill in your visa forms. Your Cambodian visa will cost you 1,000/1,100 baht, at times much more expensive than your cheap airfares to Thailand. Then cross the road for your exit stamp where, inexplicably, you will be asked for 300 baht. Walk back across the bridge to the Thai side, complete the forms for your Thai visa and get your 30-day extension.
North of Trat is the border post of Aranyaprathet on the Thai side and Paoy Pet on the Cambodian. If you want to get in and out as quickly as possible, officials are willing to help you with your entry permit and take you to the immigration office and drive you back across the bridge for a couple of hundred baht.
A couple of things to be aware of: there are many touts operating on the borders offering to fill in your papers for you. Dont! And if you haven't brought a passport photo, it's going to cost you a further hundred baht.