Ko Yao Noi
From Bangkok, it's an hour-and-a-half-long flight to Phuket, which is serviced by low-fare carriers Air Asia (airasia.com), Nok Air (nokair.com), and One-Two-Go (fly12go.com). Tickets are as low as $7 each way. Another $7 gets a taxi ride from the airport to Bang Rong Pier, about 20 minutes away. From there, boats to Ko Yao Noi depart nearly every hour between 9:30 a.m. and 5 p.m.; a 9:40 a.m. speedboat cuts the 60-minute travel time in half.
Trat is the gateway to islands in the Gulf of Thailand; it's an hour's flight from Bangkok on Bangkok Airways (bangkokair.com, from $53). Boats to Ko Mak leave four times a day from the Laem Ngop Pier in the nearby town of Laem Ngop; the first departure is at 9:30 a.m., and the last is at 4 p.m. (50 minutes, $13).
Boats to Ko Kut leave three times daily from the Laem Sok Pier in the town of Laem Sok (near Trat)—at 12:30 p.m., 1 p.m., and 2 p.m. ($11 for the first trip; $18 for the others). Mark House Bungalow runs boats at all of those times, as well as an additional 9 a.m. speedboat from Dan Kao Pier that costs $18; all of Mark House's boats require reservations (011-66/8-7059-4507). There's also daily service between Ko Mak and Ko Kut at midday (one hour, $12).
Koh Rong Saloem
Mekong Express (011-855/23-427-518) and Phnom Penh Sorya (011-855/23-210-359) run air-conditioned, comfortable coaches from Phnom Penh to Sihanoukville (four hours, from $4). A new national airline, Cambodia Angkor Air, is finalizing plans to fly from Phnom Penh to Sihanoukville next summer (cambodiaangkorair.com). From Sihanoukville, take Lazy Beach's boat to Koh Rong Saloem; it departs every day at noon (two hours, $10).
Kep is a four-hour, $4 bus ride from Phnom Penh with Hua Lian (011-855/23-223-025) or Phnom Penh Sorya (011-855/23-210-359). To get there from Sihanoukville, ask your hotel to book you a private car for the nearly two-hour drive, which costs $30 and up.
Vietnam Airlines runs several flights daily from Ho Chi Minh City (vietnamair.com.vn, one hour, from $75).
Con Dao Islands
VASCO Airlines operates daily, 60-minute connections from Ho Chi Minh City to Con Son (vietnamair.com.vn, from $90).
The best time to visit any of the islands is from November to January, when the days are cool and dry. The rainy season (which sometimes entails only an hour or so of rain in the afternoon) hits the Andaman Sea from May to October, while the Gulf of Thailand sees the most precipitation from June to October. Some hotels, restaurants, and tour operators will close down during the wet season.
Many of these islands don't have much in the way of shopping, so make sure you've got the essentials before you go: high-SPF sunscreen, bug spray, a basic first-aid kit, and antibacterial gel or wipes, to start. It's also a good idea to bring along a flashlight, and avid snorkelers may want to bring their own snorkels and masks, as not all hotels and tour operators have them for rent.
Visitors can obtain visas in advance from evisa.mfaic.gov.kh ($25), or on the spot at the Phnom Penh and Siem Reap airports and at major border crossings ($20)—just have a passport-size photo and be sure your passport is valid for at least six months beyond the date of arrival.
Arrange for a visa at least a month prior to your arrival; apply by mail or at the nearest Vietnamese embassy. Applications are processed in five business days, but shipping times vary (vietnamembassy-usa.org, $55).
Visitors from the U.S. staying for less than 30 days don't need a visa (thaiembdc.org).
A NOTE ON BOATS
For many islands in this region, private companies run boats to and from nearby port cities. Each operates a little differently, but a few general guidelines apply. The more remote the island, the sooner you should inquire about reserving a spot on a boat. (Your hotel will likely be able to help; be sure to ask when you book your room.) For islands with several daily routes, like Ko Yao Noi, 24 hours' notice is generally enough. Payment can be made on the pier or on the boats themselves; bring cash. Even if you haven't made a reservation, you may get lucky just showing up at the pier half an hour before departure time. There are often a few boats going out at once, carrying 20–30 people each, and prices don't vary that much. Word to the wise: If you can, avoid the slower, wooden boats; they cost less, but you'll lose precious beach hours.