Hmm, you have described what we leave sometimes. Who are you leaving things for? I think of the children. I think children would like to find foreign coins, buffalo nickels, hot wheels, spent cartridge cases, and McToys. (Among other things.) We also leave custom poker chips that we had made as a sig item.
I really think kids find that kind of swag interesting.
The "best" thing is completely subjective. I don't trade often. I have traded for a compass key chain and a Hot Wheel's medal from the 60's.
Yeah, I suppose everything is pretty subjective as to what's good and what's not. I guess there's nothing wrong with the coins, cars, and McD's toys. Sometimes some of those sorts of things I've found to be quite interesting, but for the most part, repetitive. I guess I don't really mind that stuff, except when it's very worn out and used, to the point of being almost broken. In the end what interests me the most is the unexpected and unusual.
I haven't taken into consideration that people leave things specifically geared for people of different ages or interests, but it all makes sense.