Rube Goldberg Science Fair

I have a Science Fair for which I have to do a Rube Goldberg on May 15 (I know, almost 4 months away), but the current problem is not making the Rube Goldberg itself. No, the current problem is: What simple task should I do? There are so many choices - and I can hardly think of any! I probably figure it out eventually… oh! I know! Google can help me solve this!