You definitely can visit other countries ALONG with going to where you said you were going. I have used AP a total of 4 times and have always visited neighboring countries for either work or to see relatives. I have never been asked by CBP the nature of my trips or why I went where I went. If you notice the AP document itself doesn’t list the country you’re going to either. That information is for USCIS discretion to approve the purpose of travel. With that being said, I would not say I’m visiting family in Mexico then go on a trip to Europe ( that is just plain dumb lol).