My in-laws have a wonderful tradition of buying an ornament for every trip they take. Their tree now represents years of wonderful memories, and they enjoy those memories each year as they decorate it. You don’t necessarily have to find the ornaments during the trip, you can give them to each other for Christmas too. Consider writing the year on the back of each ornament so you can remember when you were there and if the location isn’t obvious from the ornament itself (or you don’t want to write on it), add a little tag to it with the location and year. I got a small tag for a hand-blown glass ornament from Stockholm from Spiffing Jewelry (on Etsy at Spiffing Jewelry) that was perfect.