Originally Posted by jiejie

I think Megatoplover is on to the right idea for 18 days, which is too short to cover much territory. I think I would also limit to 4 places (2 which are your arrival/departure cities) and structure the following way. If you are not a Certified Beach Bum, then I think you will overdose with much more than 5-6 days/nights.

Fly into BKK but try to connect to flight to Phuket. (5 nights) Phuket: a big island with a series of beaches along the Andaman coast. Pick your beach carefully according to your desires for activity vs peace, as some beaches are more frenetic and busy (esp Patong) then others. I particularly like Bangtao Beach which is a lovely long stretch of good sand with nice views. However, most of the places to stay there are upscale, the lower and mid-range places get booked out in advance for Nov-Feb season. Some of the best views are up on the headlands looking out, not from down on the beach--you'll need some wheels or sometimes public transport will get you there.

I do not think the Phi Phi trip is that great anymore. Phi Phi was once a beautiful island that was ruined by more development than the island could stand, with a lot of resulting environmental problems and overcrowding on the main area. So, manage your expectations. When you tire of the beach, I would rank as a better daytrip with better views, is a sea kayaking trip in Phang Nga Bay. Some of them you don't have to do much (or any) work, just relax. Google for Sea Canoe and their "Hongs" tour. A day well spent. These pick you up at your Phuket hotel so no need for moving house.

Fly back to BKK. (5 nights, 4 full days)
Now is a good time to do all the BKK sightseeing, shopping, nightlife, etc. After a good rest at the beach.

Fly to Siem Reap, Cambodia (4 nights, 3 full days). Or overland if you are in a thrifty mood, but it will take about 7 hours. Visa on arrival if flying in, coming in by border you can get there, but is less hassle to pick up in advance (can get in BKK).
Angkor is one of the great sites of the world, even for confirmed temple haters. Siem Reap is a small town which, though full of activity, has a pretty relaxed atmosphere and a nice vibe in the evening when everyone is back in town. Three full days here are good for most people, and the 3-day Angkor temples pass option allows you to go in and out, and mix up with other non-historical things to do, like trip to Tonle Sap Lake, etc. Due to the heat (even in winter), most people break up their day into 3 parts: 1) early morning start to the temples, back to SR for lunch/rest/swim in pool, 2) mid afternoon start for temples or other activity; 3) back to town for freshen up, dinner, and evening socializing in town. Cambodia (country and people) moves slower than the Thais, which in turn moves WAY slower than the Chinese.

Fly to Hong Kong, get a morning flight. (You may need to connect in another city, test different options for pricing). (4 nights, 3 full days)
Big contrast to Bangkok, lots of interesting things to do. If you aren't a big gambler, I'd tend to skip Macau in favor of exploring HKG.

That's a good balance of beach, 2 very different cities, and a small town with a whopper of a cultural/historical sight nearby. If you want more beach time, I'd tend to cut a day out of Bangkok and slide it south.

Air Asia (Thailand) is cheap but there are some downsides--the most relevant for you is likely to be that there can be significant delays, sometimes cancellations of flights. This is not great for somebody on a short visit time frame. They also can't interline or through-connect luggage to either their own flights or to other airlines. Look into all your options before deciding.