For a while I gave everyone a gift certificate for an animal from the
local pound. Since I also included a long caveat about why only
responsible people should adopt, most went unused, so they served as a
contribution. (These funds went direct to them rather than the city, so
I voluntarily licensed my cats, too. That may not be the case
everywhere.)

Don't usually think one should force people to contribute to one's
causes, but all's fair in love and animals.