Guelph is about an hour from Toronto, Ottawa about 4 in the other direction and Montreal is about 4 more (I think) from Ottawa.
To get to Guelph and Ottawa, you will have to drive on the 401. It is the busiest highway in North America. You do NOT want to be driving on this highway at rush hour times because it is very, very congested. Plus, we are now into our other season besides winter. Construction. Only 2 seasons here, winter and construction.
So I wouldn't recommend renting a car, in light of the fact that you haven't driven much lately.
There is decent train service between Toronto and Ottawa and I believe it continues on to Montreal. There is also train service between Toronto and Guelph but I believe is is mostly a commuter service so you might be limited in travel time.
If you are flying into Pearson, you will have to take a bus or taxi to get to the start of the subway line. Once you hit the subway, though, you can get all the way down to Union Station where the trains will leave from.
Guelph is a university town so the transit is OK. When I went to school there, it didn't run on Sundays. That may very well have changed though, since that was (mumblety) years ago. Ottawa has decent transit. I don't know about Montreal.