For international flights, if you don't bring your passport and have a return ticket, you won't get pass the ticket counter
I work and live all over the world, including the PI. You don't need a visa to get in and you can stay up to 30 days without a visa. You can get, 30, 60 and 90 day extensions in lieu of a visa. Many get around this by taking a short trip to another country and returning, which re-starts the 30 day clock. Hong Kong, Singapore and Thailand are popular places for this. Having a US passport gets you in to most countries for 30+ days without a visa.