I ran for VP and won. Every poster I had was some stupid joke totally unrelated to anything in school. Halfway through the senior year, the Prez and I were kicked out because we never attended any meetings or did anything productive for the school.
Anyway, most of mine were smart @ss humor. Offering "Free College Tuition to any Seniors voting for Jeff." "Jeff wins two out of three falls when wrestling." "Win $100 by voting for Jeff." Clipart pic of a sweet duck with the tag line, "We does not utilize animal testing. Vote Jeff."
One of my favorites was a picture of a cash register. It simply read, "This makes no sense. Vote Jeff."
Go for the smart humour angle. It will at least make it fun. My strategy was come up with as many signs as possible. I had one place where I put a copy of every different sign (I say sign, but it was just a regular printout from my printer) up. Then, I would scatter them around the school. People actually started coming by my archive spot just to see what all I had done.