I was looking through our Flickr photos and discovered an earlier visit I had completely forgotten about. We once went into Mexico rather by accident.
We were in San Diego for a coastal cruise up to Vancouver in 2006. We flew down a few days early to do tourist things such as visiting the world-famous zoo. We found riding the trolleys on, the then, $5 day-passes were a great way to explore. One took us right into Tijuana. We were surprised to find ourselves in Mexico but looked around. Our passports were still at our hotel, but we had enough ID to get us back into California.
We leave for Playa del Carmen, via Toronto and Cancun, Friday evening next week.