That's a nice thought, but personally, I'm not a big fan of creating unnecessary waste - even if it's recyclable. After all, it takes a lot of water to recycle paper/cardboard, and for such a small piece of cardboard, it's more than likely that it'll just end up in the landfill instead of even being recycled.
Personally, I think just wishing or greeting someone can achieve the same thing and still make a big difference. When I moved here, people used to wish me "good morning" if I crossed paths with them during a morning jog, and that was such a huge mood booster for me (back where I lived previously, people would just ignore or even completely avoid you if they saw you across the street). I also make it a point to greet and thank my bus driver every time I catch the bus, which can be a very thankless job, and I'd like to believe it helps.
Maybe if you want to do something more visual/physical, you could perhaps give them a high-five or a hug? Like hold up a sign saying "FREE HUGS".... although I'm not sure if there would be many takers in this post-covid world. If you want to so something more impactful, I'd recommend checking out some of Improv Everywhere's videos. Maybe even get in touch with them, to see if there are any agents where you live, and then you guys could organize something big.