@Warren178
Nice idea. In my opinion, as a guest I would prefer to have the option to donate to a charity. Maybe a donation box and a pamphlet explaining.
On a lighter note, we have a lot of guests who enjoy their wine/beer at the campfire. I always gently suggest that if they have returnable bottles, they are free to leave them behind, and we donate whatever proceeds to local charities. Most do leave them, but at $0.20 per bottle, it's not a huge amount, but it does help and it makes the guests feel good.