AP Physics C, AP Chemistry, AP Calculus BC, AP English, and AP World History/AP U.S. History.
If you have not taken or planned to take the above classes, I would definitely recommend taking them over the fifth year of French.
All of those classes will help you validate things. If you can get a five on those AP tests, you will most likely validate the classes here. Validation frees up your schedule and opens up some interesting opportunities for VGEP, language minors, and double majors.
The classes are difficult, yes. But if anything, struggling through them will teach you how to study, how to deal with time management, when to get help, and when to trade what you want to do with what you need to get done.
If you have taken or plan to take the above classes and still have that extra empty block, I'd recommend a computer science class. If that doesn't suit your fancy, pick something you like; it really doesn't matter if you already have the above AP classes under your belt. If you like French, stick with French. Nothing wrong with that.