So I did this and Seattle's Space Needle within one week of each other, and the CN Tower kick's the space needle's butt. First, it actually looks like a needle (unlike the space needle), but more importantly, it actually is the second largest building in the WORLD. The space needle isn't even the largest building in Seattle...
On the observation deck there's a small glass area where you can look all the way down. If you're not afraid of heights (or only mildly afraid, like me) I'd definitely recommend standing or lying on it. It's definitely a trip.
We didn't do this, but you guys should look into the eating option. Since it costs something like $20 per person just to go up there, and its free to go up there if you eat at their restaurant, it may be worthwhile to eat up there. And if you do, let me know if its worth it!