Congratulations! There are a LOT of truly great suggestons here! I especially like the idea of a coordinated group/series of stickers (by color? font/style) so people can pick them out. Please keep me on the ping list if you have one so I can order some. My car is old and scratched; I’d never know if someone keyed it. :-)
Yes Joy, if they have a distinctive color/font and people notice different sayings, they will start looking for them.
I’m thinking black on yellow. The yellow blackground can be spotted from far away, is something that grabs your attention and black/yellow is the “caution/danger” combination.
As for font, better to stick with something easy to read like helvetica.