Cryptic and poetic notes under baseboards and desks or even within the holes in the drywall that you patch, mild unobtrusive graffiti, old pets buried in the yard, etc.
I get sentimental about the fact that I'll never have access to a certain place again, and that a certain period of my life is coming to an end. The spaces we inhabit inevitably mark us (whether good or bad), so it's nice to leave a piece of you behind before it's too late. To try and resist against it all getting flattened and annihilated by the perpetual deluge of time and change. I've found that it helps with meaning making and the navigation of loss.
Also, I think it would be fun to discover this sort of stuff as someone else. Sort of like a little low stakes mystery or impromptu archeological dig. You know, real fuel for the imagination amidst the monotony, while also encouraging you to embrace our shared existence and connection throughout time and space, concepts that are so often obscured within our modern fetishized and individualized world.