If you are in Europe (in the Schengen area) your passport is only stamped once and you can go around neighboring countries. I myself went to a second country that I didn't list on my AP application and I had no issues coming back. I had a layover for about 9 hours in Europe and the airline offered a free tour of the city while I waited for my next flight. I went to passport control as well. Coming back to the US, I wasn't asked a single question but totally depends on the officer you get.