If you're planning to drive in Thailand, you must get an International Driver's Permit (IDP) before you leave your home country. Also, make sure your travel insurance covers you in a rental car.
Save the boarding pass from the flight that got you to Thailand. Sometimes, they ask for it. Also, 3 weeks ago, the immigration officer asked to see my onward ticket out of Thailand. This is very rare but it happened to me. Fortunately, I had one.
You won’t get a visa on arrival. That’s for some other countries. You will get a visa-exempt (no visa) stamp in your passport good for 60 days. The day you arrive, even if it’s 11:30pm, is Day 1.