Southeast Asia is generally open to US citizens, with different degrees of visa-free access throughout the region.
Most countries in Southeast Asia allow fairly easy visa-free entry, or visa upon arrival, for stays ranging from two weeks to three months. The only exception is Vietnam, which requires US passport holders to get prior visa approval at a Vietnamese Embassy or Consulate.
So if you're planning to hop from country to country in Southeast Asia, be sure you're prepared long beforehand with the proper requirements, so you don't hit a wall at the immigration gate.
Visa Requirements for Cambodia
US passport holders may obtain a visa on arrival; just provide $30 and a passport-size photo. Your visa will let you stay on for a maximum of one month. Cambodia also offers a slightly more expensive, but vastly more convenient, Cambodia e-Visa that you can apply for online, for an additional $7 fee. For more information on getting a Cambodia visa, read this article: Cambodia Travel Information.
Visa Requirements for Indonesia
As of March 2016, the Indonesian government allows citizens of 169 countries to enter visa-free. US passport holders, in particular, may get one of these, but the fee depends on how long you plan to stay (30-day maximum). For more information, read this article: Indonesia Travel Information.
To apply for an Indonesian visa while in the U.S., proceed to the Embassy of Indonesia's site for consular services to get started. The embassy will not accept cash for visa fees; only company checks, cashier checks, and money orders will be honored.
Visa Requirements for Laos
Visa on arrival for US passport holders may be purchased for $50; maximum 30 days' stay.
You'll need to present the fee along with the following requirements: an original U.S. visa with at least six months remaining validity; a fully-accomplished visa application form from the visa booth at your port of entry; and two recent passport-type photos.
Visas on arrival are not only available at Laos international airports; you can also secure them at selected overland crossings between Laos and China, Thailand, Vietnam, and Cambodia.
Visa Requirements for Malaysia
US passport holders visiting Malaysia need not get a visa if visiting for social, business or academic purposes. Visitors can stay, visa-free, for a maximum of three months. For more information, read this article: Malaysia Travel Information.
For more information on Malaysian visas straight from the horse's mouth, proceed to the Embassy of Malaysia's official site.
Visa Requirements for Myanmar
To enter Myanmar, you need a valid passport with at least six months' remaining validity and a valid visa for Myanmar. The “visa on arrival” scheme has been discontinued for tourists; only business visa applicants can get one.
Fortunately for U.S. travelers, Myanmar offers an “e-Visa” - just log onto their official site (evisa.moip.gov.mm) to apply. The e-Visa costs $50 (payable via credit card); you'll also need to upload a recent color ID image along with your passport and ticket details. Without an e-Visa, US passport holders must obtain pre-approval for a visa from the nearest Myanmar embassy or mission. Find out more at the Embassy of Myanmar's official site.
For more information, read this article: Myanmar Visa - Entry Requirements and How to Get Your Visa to Myanmar. More general information also available here: Myanmar Travel Information
Visa Requirements for the Philippines
US passport holders may enter without a visa, for stays not exceeding 21 days. Visa Extensions can be obtained from Philippine Embassies, Philippine Consulates, or from the Bureau of Immigration within the Philippines. Read this article: Philippines Travel Information.
For more information on Philippines visas, proceed to the Philippine embassy's official site.
Visa Requirements for Singapore
Visa Requirements for Thailand
US passport holders can get a stamp on arrival, permitting them to stay for up to 30 days. A single 14-day extension may be obtained.
Visa Requirements for Vietnam
US passport holders must obtain pre-approval for a visa from the nearest Vietnamese Embassy. 30-day or 90-day visitor’s visa may be obtained in the U.S. from either the Vietnamese embassy in Washington, DC if you're on the East Coast, or at the Vietnamese consulate in San Francisco if you're on the West Coast.