My contract/visa ends April 30 and I would like to do a turn around at Nong Khai. What paperwork will N.K. immigration need for me to do a simple turn around? I.E. letter from my employer, canceled work permit, tax receipt?

I am heading back to the USA and I need to do a tun around to wait for my wive's visa to the US to finish.

You will need a letter from your employer stating the date your job ends. That should be the same date you leave the country.

Since you will be crossing to Vientiane it might be better to get tourist visa (or a single entry non-o visa) while you are there.

If you are married to a Thai you should be able to get a 60 day extension to visit her at an immigration office using the same letter to cancel your current extension.