I have boxes of rocks, sand, shells and leaves from different parts of Australia; some pieces of tiles from the bathroom of our old house and the house my family lived in before I was born; broken roof tiles from my sister's old house and a piece of concrete from a football ground my family went to a lot when I was little.
We have a big rock from somewhere we used to stay a lot when I was little. It was used as a doorstop, and one day I tripped over it and pushed the doorknob through the wall.
A little naughty, but I've swiped a few Guinness glasses from different pubs in England (and additionally one from here). I have a thing for branded glasses :lol They remind me of the pubs I went to and the good time I had. Oh and a nice steak knife from a steak house after I got drunk and a friend egged me on to take it, because I liked the look of it :lol
I'm not really a momento collector. I did once take shells from a beach, and also a glass from a overpriced hotel restaurant. But nothing that had ever become a ritual. I did have a really cool coworker that collected rocks though, I've never been particularly interested in rocks myself, but he was fascinated by them, as well as older arrowheads. He even had fossilized dino poop, and petrified tree pieces.. The guy was great at sharing his enthusiasm for it all, and he would travel around the US collecting things.. he also would buy things and sell them on ebay, back when it was more popular.
I mostly just take pictures, I think if I did collect momentos it would have to have a scent, like pine needles. I know it is silly, but a simple thing like that can really bring me back to where I have been in the past. I used to have a little hemp sack of Maine pine needles, and I would love to sit it next to my desk and just reminisce about the past, also it helps in virtual environments where there is no sense of smell. It really adds to the effect, to wander around a virtual forest, and have the scent of authentic pine.. and as I say this, I realize just how ridiculous I sound :lol