So I just came up with the itinerary for my trip to Thailand in August. What do you think?
Day 1 - Arrive to Bangkok
Day 2 - Bangkok (spend the day)
Day 3 - Visit Ayuthaya (temples...)
Day 4 - Visit Kanchanaburi (floating market, crocodile farm)
Day 5 - Bangkok (spend the day)
Day 6 - Travel to Trat / Ko Chang
Day 7 - Ko Chang
Day 8 - Ko Kut
Day 9 - Ko Kut
Day 10 - Ko Chang
Day 11 - Travel back to Bangkok
Day 12 - Fly to Ko Samui
Day 13 - Ko Samui
Day 14 - Ko Panghan
Day 15 - Ko Panghan
Day 16 - Ko Samui
Day 17 - Ko Samui and fly to Bangkok at night
Day 18 - Spend the day in Bangkok and leave Thailand at night.
I haven't been to Ko Chang/Ko Kut but it looks pretty well organised, probably wouldn't want to spend any more time in transit than you have listed.
The main thing to consider is that it's getting in monsoon season which may ruin your days on the beaches. It wasn't monsoon when I went but it rained for 2 days while I was in Koh Phangan - there so wasn't much to do! But I guess this also depends on the company you have and the type of place you're staying in.
I'd love to squeeze in Chiang Mai somewhere but that's probably too hard to fit in the time period without removing something :/
I agree with Erik on th eissue of the rainy season. It may not rain all day long, even though it could, your beach time could be disrupted. Not to discourage you though. I would however think about having backup plans when it comes to travel time as the rains may delay you longer than you may expect. This will be especially so when it comes to Ko Samui and any other travel from Surat Thani by boat to the islands. Rough water could get the boat cancelled and or if they are willing to take you sometimes it just isn't worth taking the risk when it comes to your life. Maybe drastic, but it does happen.
You might want to consider not going to Ko Chang. Do a few days and do Ayuthaya and Kanchanaburi, hen head south. That way you can it would save you a travel day and having to go back to Bangkok. Ko Chang is nice but not as nice as the beaches in the south, at least in my opinion, and I think most people that have spent any amount of time in Thailand would agree. Kanchanaburi is nice too but better in Oct Nov because of the waterfalls.You run the risk of the waterfalls being flooded and not as nice to see.
Anyway, have fun either way. My Phen Rai Krup!!
Bottom line, Thailand is great any season any time of the year!!!!