The first time my husband and I traveled to Spain together, we were on such a tight budget that we couldn't afford to buy many souvenirs to bring back to our kids. We bought some cheap maracas and a bullfighter beach towel and called it good. But to our surprise, the thing our kids loved most was a small rock we brought back from the shores of the Mediterranean. It was this rock that they chose to bring to school for show and tell and took next door to show their friends.
Find meaning in unexpected things
That little white rock started a tradition that has become an integral part of our world travels. Every time we visit a new place, our kids work together to find a unique rock to bring home. These are labeled with a sharpie and go into our special rock jar on the mantle. Over 10 years and 17 more countries we've collected a lot of rocks, and the jar has become a visual reminder of all the adventures we've had as a family.
Find things that will encourage memories long after returning home
Other friends and fellow travelers have their own great travel traditions. My sister and her husband always buy a book related to their destination - Shakespeare in Stratford, Lao Tsu in Beijing. Many of our friends buy Christmas ornaments distinctive to their destinations, and others bring home tiny bottles of sand.
Key chains, playing cards and refrigerator magnets are other low-cost souvenirs that are usually easy to find in most tourist shops around the world. Some friends have built a collection of music they purchased while abroad, or even specifically from the street musicians they encounter around the world.
Some souvenirs can be created from things that are useful both during and after a trip - one friend's family purchases reusable grocery bags on their travels, others buy a special beach towel from each beach they visit. My youngest sister buys one article of clothing on each trip so she can remember her experience every time she wears it (and enjoy the reactions of people when they ask where she bought her boots, and the answer is, Ireland.)
It's not always the "things" that matter
Other, more unusual traditions involve specific activities. Families might pack the same snacks for each trip, reserve certain games or music for traveling or allow some "forbidden" food only while on vacation. For example, the only time our family eats at McDonalds is when we are in a foreign country.
My favorite travel tradition, though, is one created by the Lewis family of Cumming, Georgia. They have traveled together all over the world, and in each location they take a few seconds of video of them all dancing. Daughter Haleigh then compiles the clips into videos which are the perfect reminders of their trips. This is one tradition I am definitely going to start with my family on our very next trip.