Lee Meadowcroft:
I've started to finalise my route as follows, haven't decided on
places to stay yet because I need to decide on my budget:

Sun - Arrive 17:00
Sun/Mon - Toronto
Tue/Wed - Niagara Falls
Wed - Nr Toronto
Thu - ...

Dave Smith:
Just a few thoughts. Niagara Falls is about 1 1/2 drive from Toronto.
Rather than spending 3/4 of an hour to get to a downtown hotel, getting
settled, unpacking and then having to pack again, go directly to
Niagara Falls and get the travel out of the way.

Agreed, if you're flying into Toronto and renting a car immediately on
arrival rather than later. At the airport you're already on the west side
of the city (it's just outside the western city limit, in Mississauga),
so you can get to the Niagara region without driving through Toronto.
Leaving the airport, take highway 427 south about 2 km to highway 401;
then 401 west about 6 km to highway 403, which also goes west.

After about 20 km, highway 403 merges onto the Queen Elizabeth Way
(QEW), which is signed British-style with destination cities instead
of the normal way with directions. At this point you are on the QEW to
Hamilton, which then curves around the end of Lake Ontario and becomes
the QEW to Niagara Falls. At some point highway 403 splits off again,
but now you ignore it and stay on the QEW. If Niagara Falls is your
destination, highway 420 is the usual exit.

(An alternate route from the airport is to stay on 427 south to the QEW,
but in my experience there tends to be more traffic that way.)

These are all freeways with 100 km/h speed limits and traffic usually
a bit above that. There are also back roads with lower speed limits
that will provide scenic alternatives with lighter traffic, when they
aren't going through cities and towns. You might want to try one of
those routes on the way back, although they can be trickier to navigate.
--
Mark Brader, Toronto "People say I'm a skeptic --
but I find that hard to believe."

My text in this article is in the public domain.