When my son was young, we traveled quite a bit. My husband is British and we moved across the country causing us to travel to visit family and our new home in California. Before he was 5 he had been to Los Angeles, London, New York and DC in addition to our family vacations. As a result, I had what I called a Mary Poppins bag stuffed with all kinds of toys and activities to keep my son occupied as we traveled around. On one visit to Los Angeles we were staying downtown. To keep Rowan busy one afternoon we went up to the rooftop of our hotel and played City Scavenger Hunt. Scanning the area below we would look for things like digger trucks, pigeons and construction sites.
This is an updated version of that I posted about when my son was little resized so that they fit neatly in a mint tin. I also added circles so that kids can check off the item once found. You can print out multiple copies and store them so that siblings or friends can have a little competition.