I arrived in Thailand on the 5th of September (I am american) on a 30 day Tourist Visa (the one that they issue you on the flight and you filll out). Do I just need to leave the country for a day or so before my 30 days is up and return on another 30 day tourist visa? I am planning on staying here for a little under 2 months. I was planning on going to Siem Reap, Cambodia for a day or two. Will I just get a tourist visa on the return flight?

That's not a visa, you have a "visa exempt" entry. Yes, just leave the country before the 30 day expiration of your stay marked in your passport. Upon reentry you should get another 30 day stay.

The correct Term for the 30day stamp is "Visaexemption" or "Visawaiver".

Upon entry you will be granted a 30day stay, if you depart for holiday to Cambodia and return you will be given another 30day stamp.

Be aware that at some landborder crossings you will be asked to show a confirmed onward travel ticket.

And the limit is a total of 90 days in a 6 month period using Visa Exempt Entries.