1. Write a letter they enjoy. Outright humor doesn't work in a cover letter. It generally can't be pulled off. It falls down because someone is always the butt of the joke, and in an interview situation you don't want that person to be either you or the recruiter. Rather than telling jokes, connect to the hiring manager in a light hearted way. Try _ "I was hopping from project to project so often I thought my legs had springs." Then make the connection to the reader. "I'll bet you can relate to that." And finally, segue to the value you can add. "I tried a lot of remedies until we finally developed a scheduling system that actually created more productivity and less useless movement. I'd be happy to tell you how we did it and see if something like that would be valuable for you."
Make sure you choose a cover letter format that you feel most comfortable with. It should be something that could best express your personality but keeping it as formal as possible. Be wary of the details of the cover letter format. It must be clean and neat. It should provide the data directly. There is no need to be colorful or playful with the fonts.