Any suggestions for Thailand Travel Appreciated!

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Hi -
I am going with 2 friends (guy, girl) to Thailand in Aug. We will be there 10 days.
We thought about flying into Bangkok, going to Chiang Mai and then to a beach location (or vice versa).

1. What's the best airline to travel from states to Thailand? We've concluded we're not flying an American airline (since other airlines offer more room - imperative when in the plane for ~25 hrs)
1a. Did you find it's best to fly into Bangkok?
2. Beach area you suggest? Sounds like Phuket and Phi Phi are too $$ at the moment and too 'crowded' (though not sure what thailand terms 'crowded'? Are we talkin like club MTV Spring Break Cancun?)
3. Hotels you suggest? We're late-20s (no high rollers here!) and looking for best deal w/o compromising safety, enjoyment, etc.
4. Suggestions for inter-country travel - most economical and most logical - among Bangkok, Chiang Mai and a random beach town?
5. Any must-sees in your opinion?

I know these are a lot of questions so any input would be appreciated. Thank you in advance for advice you have.

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