Southeast Asia forum

That really is a lot to try and fit into 3 1/2 weeks, you'll get the sample platter - but won't have much time to try a second bite if you really like something.

Thailand is big - so it depends on what you want to see. Just doing Chiang Mai in the North for a few days and then a few days on an island and you've already filled up a week. Give yourself another week for both Phnom Penh & Siem Reap . . . then fly down to Bali and skip Singapore . . . or skip Cambodia and spend few days in Singapore, but I'd skip one for sure.

Since you'll certainly get a lot of country, temples, and culture in Thailand & Bali you may want to skip Cambodia for now so you can get the megacity feel in Singapore - and since Singapore wouldn't need as much time as Cambodia you'd have more time to fit into Thailand and Bali.

Travel time from Bangkok to the Islands is at least a day each way, same with destinations in Cambodia, and flights to and from Bali/Singapore will usually eat up the better part of a day. You've always got to figure that into an itinerary.

Cambodia to Bali is not convenient -- you'd need to fly and connect in one of the cities already on your itinerary (Singapore, Kuala Lumpur, or Bangkok).

You arrive in KL? I'd suggest something like this:

Kuala Lumpur to Singapore (by road), Singapore to Bali (fly), Bali to Phuket (fly with AirAsia), Phuket to rest of Thailand ( Chiang Mai and Bangkok), Bangkok to Cambodia

Note that there isn't actually a train station in Phuket -- the nearest one is in Surat Thani about 200 km away.

I know there's already a lot of flights in your itinerary, but another that would save you a lot of time is Phuket to Chiang Mai. Then you could take the train from Chiang Mai to Bangkok and still see the countryside. Otherwise Phuket to Bangkok is about 12 hours and Bangkok to Chiang Mai is another 10ish.

There are good flight connections from Singapore and Kuala Lumpur to Cambodia, so an alternative would be something like this:

KL to Bali (fly), Bali to Singapore (fly), Singapore to Cambodia (flights to Siam Reap or Phnom Penh), then overland to Thailand for whatever time you have left.

KL to Singapore is only about 4-5 hours by bus, including immigration time. Very easy and do-able.

Most KL to Singapore buses are about 30RM, but there are luxury buses with WiFi/meals/etc. that cost more.

For Bangkok to Siam Reap, check out this Travelfish feature about all your options -- http://www.travelfish.org/feature/71