I live in Toronto and think it's a great place to visit! My favourite times of year are late spring (May) and early fall (September, early October). It's usually sunny and warm then, but not hot or humid. The summer is also great - there's a LOT happening in the city, all kinds of festivals and events and concerts and outdoor movie screenings etc. etc. etc. - but it can get hot and sticky. Some winter days can be beautiful (alas, usually the coldest ones, when it's also sunniest), but some can be just blech - slushy, damp, awfulness. I wouldn't recommend winter unless you love it.
There are tons of great restaurants of all types of food. Lots of patios for sunny day drinks and great parks to have a snack and a drink in (Trinity Bellwoods, High Park in the west, Withrow in the east). Lots of great little art galleries (and the big one, the AGO). The waterfront (Harbourfront) and the Toronto Islands are lovely to walk around and give you an excellent view of the city. You might have read that they're closed this summer due to high water, which isn't entirely true! You can still go over on the ferry (less than 10 minutes and about $7 or $8) and walk around - it's just that the Centre Island theme park has been closed, that's all.
Some great neighbourhoods for exploring include Kensington Market and Chinatown, West/Ossington, Dundas West (Little Portugal), Little India in the east end, Leslieville, the Beaches if you don't mind going out way east.
It's fun to catch a Jays game at the SkyDome (now called the Rogers Centre, but it'll always be the SkyDome to me!).
Yonge Street/Dundas Square are tourist traps! Kinda fun (in a busy way) to travel through en route to something else, but not a destination in themselves. Unless you like tourist traps, people acting like moving statues, people preaching about the end of the world, that kind of thing. I suppose it does offer lots of human curiosity.