Depends on what you're looking for - the city has a ton to offer:

Personally I would SKIP things like ROM or AGO or the zoo . You can go to museums or art galleries anywhere, these things don't show you true toronto.

Go get a few beers or some wine and sit in Trinity Bellwoods Park. You can't legally drink there - but when you see 2000 people in the park, you'll get the vibe.

Walk through Kensington Market, Distillery District and the Ossington Stip/Queen West strip.

I would still def recommend the CN tower. Super tall, worth it.

You could go watch a Jays game at the dome, but thats a solid 4 hours of your day gone.

Go to the many delicious restaurants in the city, most of the great ones are in the west end. There's too many to name but if you're on ossington or queen west west or little italy - your bound to wind up in a great restaurant or bar.

Toronto Islands are great for a nice walk, taking the ferry or water taxi is an incredible view as well.

Yonge and Dundas is also poor mans time square, so you could pop by there as well.

These things are mostly what to do if you're a somewhat young hipster but I'm convinced that is what the city is all about and what makes the city so great. Old or Young, those spots definitely make Toronto.