I was planning to stay in Thailand 30 days this summer, but after talking with my wife, we decided to stay for two months.

She is Thai but lives with my in Germany. I would be entering with visa exemption, and then I would extend the visa for another 30 days at the immigration office.

Is there a problem if I have a return flight after 59 days when entering the country with a 30 day visa exemption? Do I have to show any flight ticket to a neighbor country within 30 days or just tell them that I'm going to extend the visa for another 30 days after entering the country?

I have been 8 times to Thailand and I have never been asked for proof of return ticket until now but I want to be sure, just in case, before changing my flight.

You cannot be sure as nothing is certain.  Immigration most likely (99%) won't ask for anything.  The airline may (50%) ask at check-in in Germany.  You ought to be OK with your 59 day ticket telling them you will extend and having your wife with you might be helpful, but the only way to be sure is to have an onward ticket within 30 days.  You can just buy a cheap real ticket or a rental ticket, either way won't cost much, probably not needed but would give you peace of mind.  Or buy an expensive real ticket which will get refunded if you cancel.  Or get a tourist visa (which will likely cost more than a cheap onward ticket).

1 hour ago, Upnotover said:

You cannot be sure as nothing is certain.  Immigration most likely (99%) won't ask for anything.  The airline may (50%) ask at check-in in Germany.  You ought to be OK with your 59 day ticket telling them you will extend and having your wife with you might be helpful, but the only way to be sure is to have an onward ticket within 30 days.  You can just buy a cheap real ticket or a rental ticket, either way won't cost much, probably not needed but would give you peace of mind.  Or buy an expensive real ticket which will get refunded if you cancel.  Or get a tourist visa (which will likely cost more than a cheap onward ticket).

One way ticket to Cambodia would suffice, right? It's about 50$. It's probably cheaper than paying the fare difference if I wait until September to change my flight back to Germany.

5 hours ago, phobos1911 said:

One way ticket to Cambodia would suffice, right? It's about 50$. It's probably cheaper than paying the fare difference if I wait until September to change my flight back to Germany.

Buy a "rent a flight"

Google companies such as "onward ticket.com"

 

https://blog.onwardticket.com/flight-without-plane-ticket/best-onward-ticket