Writing on U.S. currency is legal so long as it doesn't (or 'isn't intended to') make it 'unusable'. It's a medium everyone seeks, and your message cannot legally be destroyed if you use a permanent marker.
You could simply do something like Where's George or write something more interesting. Copyright infringements (DeCSS), 'secret' plans to assassinate Bush, chain-money ('Copy this message onto ten bills and give them away to strangers or you'll die alone and bankrupt.'), advertisements ('Wikipedia.org: The Free Encyclopaedia', anything. Quotes, perhaps. Andrew Jackson of the 20USD bill once said
It is to be regretted that the rich and powerful too often bend the acts of government to their selfish purposes. Put that on your bill and spend it!
I'm not sure about Canadian money. Most of the legal stuff I found is on clippings or shavings from coinage or 'mutilated', etc. coinage. One person I've talked to says it looks as if you have to hand over to the queen whatever you used to deface the coin, at least with melting down coinage. (So if you ever want to meet her...)