If you want to rent a car in Thailand, suwannabhumi airport is a great place to do this. It's international companys and you can buy insurance to cover everything. Go one floor down from arrival and you will finde several car rental companys.
Contact everything in thailand if you want us to get you a better deal on car rental. Sometimes you can save up to 4500thb just by calling and talking directly with the rental company.
When you rent a car in Thailand you always have to check if there are anything wrong with it. Acutally you have to do this no matter where in the world you rent a car. If you rent a car in The Capital airport of Thailand it's easy to pick up and deliver back.
You will sign all papers in the airport and then you are on your way. The rental companys in Bangkok airport are fast and they have a high service. The search engines we listed also do searchs for Chiang Mai airport, Koh Samui Airport and Phuket Airport.