I've never gone, so I can't say for sure. But to my understanding, all you have to do is buy two plane tickets (each way). One to Toronto or Mexico City, and then a completely seperate flight to Havana, Cuba. When you arrive, most times they'll put a little piece of paper in your passport and stamp that (instead of the passport itself). All you have to do is hang on to that little piece of paper.
I'm not sure what kind of trouble you get in trying to get back to the states if you have a Cuba stamp? As I said, I've never gone, and I heard this info awhile ago, so there may be more updated travel info that I'm not aware of. I think it's easier to get there these days, but I'm not sure if any US based airlines fly there.