I've lived in Toronto for 28 years, so here's my advice. Skip Algonquin as it is a 3-hour drive from the city and not really suited to day tripping. Niagara Falls is easy to do, especially if you have a rental car. There are tours as well, and you can even go by bus if you don't mind walking a bit from the bus stop. If you can, make a stop in Niagara-on-the-Lake, a quaint little town nearby, known for its wineries.

Of course you should spend some time in Toronto itself. I wrote a post about things to see and do for people on a short layover (arranged by interests) but you could string some of these together to occupy a couple of days: http://bigtravelnut.com/toronto-layover-what-to-see-and-do/. If you like walking tours, I recommend Urban Adventures:  http://www.torontourbanadventures.com/Toronto-tours. Or read this post for more ideas: http://bigtravelnut.com/walking-tours-in-toronto/.

Hope this helps!

I was in Toronto for a conference (arriving Monday and leaving Friday), so I was tied up during the day, and I did not have access to a car so I couldn't visit Niagara Falls.  And I was by myself.  So I decided to have lunch and dinner in various places where it wouldn't matter so much that I was alone.  I took a narrated boat tour of the lake (although it might be too early for that - I was there in June), and had lunch at the Boathouse Bar and Grill (where they were filming a movie) and ate dinner at Queen's Quay.   I also had dinner in the CN tower one evening.  There are tour buses, and I didn't take any of them, but I rode the various buses that picked us up at our hotels and took us to the convention center.  I also played hooky and took the "wives tour" while I was supposed to be attending seminars.  There is a City Hall Tour, the Hockey Museum and a Sugar Museum (which I didn't go to).