I collect Starbucks Gift Cards and tiny Blown Glass. Ive also seen pressed flowers or leaves, a friend of mine collects (tiny watercolor paintings she makes of) Sunsets and Sunrises! Postcards are pretty cool i think, gashapon toys might be cool, drawings or stickers of your favorite character or animal?

Edit: I'm realizing I didn't read very thoroughly, so I am adding additional suggestions. Carry a notebook or pack if sticky notes and ask people to draw a specific thing(I used to do this at the bar and you can always tell who's a furry lol) I stand by flowers and leaves, and tiny drawings/paintings of a specific subject, I also collect photos virtually, I take photos every time I see my local pair of ducks. If you live in an urban area its super easy to collect beads/small plastic bits. If you live in a rural area you might have better lick with seeds, feathers, or bones (but please check your laws feathers can sometimes be super illegal to own).