I was just there last week in Toronto and I must say there are a lot of places that make it a fun and interesting drive.
Definately hit up niagra, i didnt manage to get there but it was so close i could taste it, i ha to turn around and come back cause i didnt want to miss my flight. I dunno if I'll ever make it back so thats probobly the biggest thing i missed.
Walking tour of downtown toronto was great. Its a bit cold but the buidlings, architecture, and the CN tower - $#@ amazing. Do the CN at night, do the rest by day, it makes a heckuva difference. And just so you know there is an "underground" mall underneath downtown so you dont have to walk outside during the evening but frankly i walked around at 10 at night and felt safe enough but rather cold.
After toronto i liked driving up to the city of oshwa. There are some really neat rolling hills right outside the city of oshwa with some great horse farms (one a triple crown winner was born at) and it was just too cool to see the horses playing in the snow. Also lake ontario from oshwa was neat to see as there park from there really has a neat openness to it along with a small little lighthouse that makes it a neat site to behold.
Beyond that i didnt see much but just as a final note driving in canada took some getting used to. People were politer than im used to (driving in AZ and CA) moving over and leaving the left hand lane open. ALWAYS stop at right hand turns, no rolling stops. And stay outta the right hand lane unless you plan on pulling over or stopping quickly. I found myself too many times slamming on my brakes because for some reaon they will pull over and park at completely random times. or in the right hand lane if the snow bank is in the usual parallel parking spaces.