Hi Duane.
I live in Singapore and have travelled quite a lot in Malaysia & Thailand.
Couple of suggestions.
For the islands, consider Langkawi (NW Malaysia) first. From Langkawi you can then get a boat to Ko Ra Wi and Ko A Dang. They’re much more unspoilt and undeveloped that Koh Samui, Koh Phi Phi, Phuket, etc. Your Baht will also go further.
If you can time it right, it is still worth seeing a full moon party at Koh Phangan, especially if you head on north to Ko Tao afterwards. Don’t be lulled into a false sense of security with regard to drug use though (if that’s even your thing), mushrooms are legal but everything else is most definitely not.
If you do get to Chiang Mai consider heading on north to Pai (pronounced Pie), it’s more unspoilt and relaxed than Chiang Mai.
Be careful at the border of Malaysia & Thailand. Southern Thailand is potentially dangerous so keep your wits about you. I’d personally avoid the far south of Thailand by taking the ferry north from Ko A Dang.
If you’ve got any specific questions feel free to drop me a line (email in profile).
Ben