Our school does a poster contest for anyone who wants to participate. A gift cert. is arranged by a local merchant. It is a nice way to decorate for an assmbly too. You can even add to it and have each child who attends the assmbly be the voters by hanging an envelope under each poster.... then giving each student a ticket and when the kids go in for assembly they drop their tick in the envelope.
Check out this site for some really neat ideas to get your school AND community involved:
web page
Your state health department may be willing to donate free anti-drug coloring books to your younger students, state police or local police often offer a "free" anti-drug program...Good Luck!