Koh Kong has an airport but it's not currently used for commercial passenger flights. Runway is in a less than desirable shape (May '08) and airtravel will not commence commercially for a while.
One way to get from Sihanoukville to Koh Kong (or vice versa) is the fast ferry (USD25-30 for foreigners) running, as of March 2009, every other day, which leaves around 08:00-08:20 and takes 4-5 hours - however this option is not the best if the weather is less than favorable and there's a heavy swell. Boats have a tendency to be extremely smelly and noisy inside (dieselfumes, khmerkaraoke) - travel on the rooftop is fun in nice weather, don't forget to apply a good sunscreen.
Please keep in mind, that these bulletstyle boats have no or very limited safety features as required by other nations public transport vessels. Their only exit is a mere manhole (a steel door) in the enclosed hull of about 120 by 65 cm.
Koh Kong is also linked to Phnom Penh and Sihanoukville via Sre Ambel by National Route 4. The road is now completed including all 5 bridges (May 2008). It's a good scenic drive through some of Cambodia's least developed and unspoiled regions - Cardamom mountains. Minibuses and tourist air-con buses leave Koh Kong mainly in the mornings (but some do even in the afternoon, probably depending on number of passengers available). On the way back, they usually leave in the morning and arrive to Koh Kong in the afternoon. Expect to pay about USD8-12 (USD8 or 10 is a price often asked in Sihanoukville travel agencies for the ticket back to Koh Kong).
Currently (Jan 2010), there is a daily bus leaving to Koh Kong from Kep for 20USD. It leaves in the morning at 8:30.
The Thailand border crossing named Cham Yeam is about 9 km (by the road) northwest of the town. Motos (normal price is USD1-1.5), shared taxis and taxis (USD2-3 for the entire car) go between the border post and the town/port, and the drivers of these will attack you while you're waiting for your visa. You may and will be asked for much more (200 baht is quite common) - just bargain a lot, and do not let them to rip off you. Additionally, you'll have to pay toll fee for the bridge, which can be paid in either baht, riels or USD. The ongoing rate will be stated on the toll booths just before the bridge - better pay this on site and do not include in your taxi fare, to avoid a chance of being over- or double-charged. When in town, the driver will offer you to exchange some money to riels - just refuse, the rate would be very poor, USD are accepted universally in Cambodia, and, in Koh Kong, Thai baht are accepted too. If you're continuing on to Cambodia, he'll also try to convince you to buy a bus ticket for an inflated price, USD15-30 or more - just do not agree, or you'll be charged this price on arrival to the agency, and bargain when he drops you there. In the agency, you may be told that the normal price now is USD25 or 30, and, as such, USD15 is a big bargain - this is not true.
Around Koh Kong, there are islands, mountains, jungles, rivers, waterfalls, a zoo, casino, and a lot more.
Koh Kong Island (considered one of the best beaches in SE Asia). There are no accommodations on Koh Kong Island as it is military run and therefore illegal to spend the night, however day trips are offered by many guest houses in Koh Kong.
Along highway 4 (Sihanoukville - Phnom Penh), about 30 km south of Traeng Trayoung, 20 km north of the Koh Kong turn-off, there is on the west side of the road a small turn-off which leads to a nice set of rapids called O'bak Retes. Great place to stop.
Day tours and overnight adventure trips to Koh Kong Island and other Islands in Kong Province, Pream Krasop Mangroves(Asia's Largest), Waterfalls & Trekking, 4WD Cardamom Mountains are available from Koh Kong Divers
Scuba Diving has now taken off in Koh Kong, Koh Kong Divers offers PADI courses and daily dive trips & snorkelling to the best of Cambodias dive sites. http://www.kohkongdivers.com
Fat Sam's bar on the main street in the downtown area is a fabulous place to get an idea about what to do - Fat Sam himself is very helpful and knows the place quite well.
Fat Sams,The Blue Gecko Bar
The Aqua Sunset Bar & Resturant has just opened 01/11/09. Riverside bar and restaurant set over the water on the Koh Pow River, pleasant river breeze, popular sunset venue. Full bar facilities, selection of spirits and cocktails, western and thai-khmer food. Also offering a Sunset Cruise every evening.
The Dive Inn. Daily specials from 110 Baht, European, Thai & Khmer cuisine. the lagers selection of spirits and cocktails in koh kong
There are many guesthouses and small hotels in the central part of town and on the riverside street, partially thanks to Thais who come to Cambodia for gambling (the big casino can be seen on the way from the border). Basic guesthouse room with fan costs USD3-6 (or 100-200 baht). For USD10 and more you'll get an air-con room in a guesthouse or hotel. As often in Asia, the cheapest places and room rates are usually those found "on the spot" on arrival. Motorbike taxi drivers will be happy to deliver you to a guesthouse/hotel of their choice, if you're not sure where to go - just ask them (and notice after while riding there) how far from the center is it and what's the price. Alternatively, just walk around and look for accommodation yourself - the town is quite small.