When I was younger and literally traveling on a shoestring budget, traditional souvenirs were too expensive so I came up with some no cost options.
One of my first collections was pictures of streetlights. It seems like every city has its own streetlight style and some of them are really unique and fun.
On this trip, I also collected pictures of manholes. The Japanese artistic district manholes were truly stunning.
Another friend collects pictures of doors and doorways. As you might imagine, she has found some beautiful examples.
If you want mementos that can be placed in a scrapbook, train and museum ticket stubs are great.
In Japan, most chopsticks come with beautifully decorated paper chopstick covers that would make great add ins to a scrapbook page.
The only limit to cost free souvenirs is your own imagination. What will you collect?