If you are on one of the Trat islands - Koh Chang, Koh Mak or Koh Kood - you will probably want to go to Cambodia as you are very close. The best option is to go overland, it's the cheapest and there's no need to go back to Bangkok.
How to get from Koh Chang, Koh Mak or Koh Kood to Siem Reap?
The best option we found was to take a ferry to the port, followed by a minivan and a bus. We got this transportation through a travel agency on Koh Mak, the smallest of the three islands, for 1,000 baths (including the ferry). It is likely that an agency on Koh Chang or Koh Kood also offers this service.
If you do not find this option, we recommend that you take the early ferry from the island to the port and, once there, talk to a tuk tuk or cab to take you to Trat or to a travel agency where you can get a transfer to the Cambodian border. The closest agency to Siem Reap is Aranyaprachet - Poipet. Below, we leave you a picture with the prices and the routes that the agencies do from Trat to Cambodia:
To enter Cambodia it is necessary to apply for a visa that allows you to stay in the country for 30 days with a single entry.
On Arrival Visa: The on-arrival visa is only 30 USD and can be obtained either at the Cambodian airport on arrival or at the land borders. All you need is two passport photos and 30 USD in US dollars.
E-VISA: You can get a visa from online here. It costs 30 USD + 6 USD processing fee. It takes 3 working days to be approved.
If you are going by land we recommend you to read the most common visa on arrival scams and we tell you how to avoid them.
The infamous Cambodian Visa scam at the border
Once the minivan arrives at the border, you will get off with your backpacks and some people who look like they are from the same company as the minivan will come up to you. They will tell you to leave your backpacks there, ask you if you have a visa, and if you don't, they will hand you a piece of paper or ask you for your passport saying that they will help you get the visa.
DO NOT CASE. Tell them that you see them on the other side of the border. Ask them which bus you have to take on the other side, ask them to give you the sticker to get on the bus on the other side of the border and follow the road alone. They will probably tell you that if you do it alone you won't make the bus and you'll miss it, or that you'll have to wait 5 hours to catch the next one. Don't believe them, just keep going.
They want to "facilitate" the process and get you the visa, so they will charge you 50 USD or more instead of 30 USD, which is the cost of the visa. At no time do they tell you that they charge you more than what it costs, nor that you have the possibility to do it for less money, it is their business. Be careful, and don't confront them because they get quite violent. Calmly tell them that you are doing it on your own and that you see them on the other side of the border. Surely they will insist, we knew all this beforehand, but they made us hesitate and we almost stayed with them, but no! You go on.
We were told that there was a long delay at the border and that if we did it alone we would miss the bus. The border was EMPTY! We were almost the only ones crossing and obviously we arrived on the other side 1 hour before the rest.
Visa on arrival
Once you cross the border into Thailand, there is a checkpoint to get a Cambodian visa. If you already have a Cambodian visa, go straight through to immigration. If you don't have it, go through the checkpoint. The visa costs 30 USD, don't let them make you pay more. And, if you don't have a passport photo, they make you pay an extra 2 USD or 100 baths. The procedure is very simple. Once you get your visa stamped, you go on to immigration.
Once you have signed your passport, you have to find your bus to continue your journey.
Another famous scam at the border is to be told that your bus is late and that you will have to wait several hours. Here comes the scam, they offer you the alternative of taking a taxi for about 30 USD to get there faster. Sometimes it is true that the bus is late, but it is up to you to pay the extra 30 USD or wait for the bus you have already paid for.
If you decide to take a taxi, don't take it right there at the border, walk a little further and get a lower price.