That's great that the students are enjoying the cards.
The city stamps are fun. I actually end up teaching a bit of social studies with those as well. A lot of my elementary school students have heard the names of the cities, but they don't know which country they are in. Some don't know where those countries are. I also like to ask them what the country is known for. Most of the time they answer with the food from the region.
The passports are on the
High Town cards for those that are interested. There is a lot you can do with the cards and the information on them. The High Town game page has previews of the cards and sample target language ideas you can use.