My advise:
Go straight to Siem Riep. Get it over with. Fly out the same day you arrive, or the next morning.
It's awesome!!!!!
And will leave you with lots of down time (evenings and nights) to recharge and recover from jet lag.
2 or 3 days is enough.
Fly back to Thailand
that evening, after seeing the ruins.
Now, THE REST OF YOUR TIME IS IN THAILAND!
The next morning go straight to Koh Chang.
Then, WHILE YOU'RE ON THE BEACH, that's when you decide to stay shorter or longer there. You may love it and want to spend a week there.
You're close enough to BKK that if you decide that you want to fly somewhere else, you can. But 16 days isn't that long.
FILL IN THE MIDDLE SPONTANEOUSLY... airfares are cheap enough ... JUST WING IT!
NOW THAT's A VACATION!
Last 3 or 4 days in Bangkok.
A nice hotel, City sightseeing, and Shopping.
My advice is not to cram too much in the first visit, There are enough tourist sights in Bangkok.
Summary:
Siem Riep
Koh Chang
(fill in the blank) flexible time to stay or move on!
Bangkok
Anything you missed... save it for the next trip!
