Besides being a fascinating piece of information for a polytheist, those coins on the walls in Santiago also prompt me to ask myself what do I do to honour the wights of my home and city, the genii who dwell or preside over buildings, gardens, isolated trees, hills, and rocks, riverbeds and mouths, streets, crossroads, and alleys. Those who live in the same place I do and are, in the end, my neighbours. Before going to Galicia a year ago, I occasionally poured water on trees, but ever since I came back I’ve been increasing the kind and number of gestures of respect towards the genii loci of Lisbon. Apart from pouring water or other beverages, there are several other small daily gestures one can do: leave flowers on places that might have a wight, scatter a portion of wheat, hang biodegradable ribbons on trees, leave a rock, or simply salute Them by kissing the tip of your fingers before touching the residence of the wight.
And yes, put coins in wall cracks, though only deep ones where people can’t retrieve the offering. One, two, or five cents coins just like in Santiago.
P.S.: One point that usually comes up in this topic is the question of animals eating the offerings, be it strayed ones or natural fauna. In my view, that’s perfectly alright: those animals are local residents, too, and some of them might be close to the wights, if not a form of Them. Just be sure to leave food that won’t harm them. Especially in places like beaches and woods, always leave eatables that are as natural and organic as possible so it won’t affect the diet of the wildlife and the general ecosystem (which also means biodegradables are mandatory!).