I used to live in Toronto, and made the trip to Jacksonville a few times. It is a pretty solid 24 hour drive, (we used to drive it straight). Now I would agree with others that you need two days there and two days back.

As far as Niagra Falls in April, it is a littel cool but definately Spring in the wine region, which is Niagra and area as everything is greening up by then.

Toronto is close to an hour from Niagra Falls. Worth going if you want to see a show or event there but otherwise you probably wont see much in your time frame. Even though it's only an hour, its usually a couple weeks behind in the season.

Let me know if you have specific things you want to do and I could suggest some things. I will assume since you will be in a wine region you will take in a couple wineries? There are lots there.