I am not a 'bumper sticker' kind of person but I do notice them. Here are some that I liked:
"I'm not really a 'bumper sticker' kind of person."
"I could go either way on a lot of things."
"My other car is substantially similar to this one."
"I support one of the leading political candidates, but that's all I'd like to say about it for now."
"The benefits of environmental protection measures should be thoughtfully evaluated and the sound ones enacted."
"In most contexts, "what would Jesus do?" is not a particularly helpful inquiry."
"Yes, my child may be on board, but I recognize that you are already driving as safely as can be expected if only for your own protection."
"I don't feel strongly enough about any particular musical performer to put a sticker extolling him or her on my car."
And my two all time favorites: "Don't blame me - I'm from Massachusetts." and "BUCK FUSH."