I find that most people in my life have at least one "thing" they are collecting. It is often an insight into their personality and it is fun to hear people talk about something they are passionate about.
I have a lot of small bottles and vials waiting to be filled. It started decades ago when I found a quaint little glass tube with a stopper in an antique shop at my favorite beach town and thought it'd be nice to take home some sand from my favorite spot, thereafter becoming a tradition for subsequent visits. Now I pick one up whenever I see one to have a cache available for any future adventures.
It's super easy to just collect then when you see them (even at craft stores and plant shops) and then it ends up kinda encouraging you to think up adventures so you can fill them.
I've got some with sand, dirt, ocean/lake water, gravel, some with just air (captured a cloud/fog while high atop a mountain), tears, paper, burnt campfire wood, etc.
My secret sauce is that I don't label them so I have to try to call up the origin story in memory and I get an excuse to tell a tale whenever someone asks me about one.