i wouldn't be too bothered with northern thailand (chiang mai etc). it's fine, but if only going for a couple of weeks then stick to the beaches.
agree on the cost - when i went there first about 7 or 8 years ago the exchange rate was around 70 baht to the (sterling) pound. it's 40ish now. it's roughly the level of portugal now cost wise excepting accomodation which is still relatively cheap.
would go along with gil on phi phi, probably my favourite of the islands over all.