I am flying in to Thailand and have a return flight two months later out of Thailand. Tourists from Canada flying overseas are aloud 30days max when entering, which is granted upon arrival. Will I have any problems entering if my flight is past the 30 days but I plan to leave before the 30 days to visit neighboring countries? I won't have any proof to show I will be leaving Thailand since I won't be flying into Cambodia, etc.
Technically you need proof, but it is rare that immigration asks for it. Dress decently and you will probably cruise right through. The only folks I have seen having to prove are those dressed shoddily (hippie, rasta, poor backpacker styles).
Keep in mind that you will only get a 15 day entry permit if you come back to Thailand overland from Cambodia, Laos, Myanmar or Malaysia. You only get the 30 day permit when you fly into Thailand.