CNN is a really interesting tour! It's something you can remember when you get back home, because every time you turn on CNN you'll recall seeing the newsrooms, the backdrops, the desks, etc.
A really fun restaurant that is absolutely unique is The Varsity. It's a crazy kind of place, not fancy at all, and if you go you should google it to learn a little about it. Because when you get to the counter and hear all the servers yelling "whattayahave" it can be a little dizzy-ing! But what a fun place and what great food!
The World of Coke is also really fun. To see those fountains dispensing coke from across the room and trying all the fun foreign-country flavors, is interesting.
The zoos and things are great, but if you only have 6 hours, I'd suggest CNN because you can't find something like that in many other cities, and there are so many zoos and aquariums in other places.