There are many possibilities for day trips from Toronto (within a 1-3 hour one-way drive). Some destinations can be reached by bus or train, but if you want convenience, go by car. Some destinations are better from May to October, while others are great year-round. Let’s deal with the possibilities by compass direction from Toronto:
Due north is Orillia, a pretty town on Lake Couchiching made famous in Stephen Leacock’s (Canada’s Mark Twain) book Sunshine Sketches of a Little Town. There is now a casino there too called "Rama," if you are so inclined.
Further north is the start of Muskoka Cottage Country with Gravenhurst, Bracebridge, and Huntsville, in that order, from south to north. Northwest of Toronto is Wasaga Beach, Collingwood, Thornbury and Meaford on Georgian Bay. There are small villages to explore off Highway 400 between Toronto and Barrie as well.
Due east off Highway 401 are Port Hope, Cobourg and Grafton on Lake Ontario – great for quaint shops, good food places, antique-hunting and pleasant walks. Further east, but possible with an early-morning start, is Belleville and the Quinte Peninsula that has Picton as its centre, and the wonderful Sandbanks Provincial Parks with its wind-swept sand dunes.
Northwest along the 401 highway and then either 86 or 7&8 is Mennonite country and Festival country. The former includes the towns and villages of St. Jacob’s (market days on Thurdays and Saturdays), Waterloo, Elmira and Elora with its pretty gorge on the Grand River. The town of Galt (now known as Cambridge) is also very interesting and scenic. Festival country nearby includes Stratford (reservations a must!) where Shakespeare and other productions flourish from May-October, and the surrounding villages such as Shakespeare and St. Mary’s.
More southwest, around the bend of "The Golden Horseshoe" of Lake Ontario is Wine Country and Niagara Falls. Use the Queen Elizabeth Highway (QEW) that parallels the Lake to get there. Wine country is located in the shelter of the Niagara Escarpment and includes the pretty city of St. Catharine’s with its farmers’ market and scenic marina.
The wine/fruit belt towns are Grimsby, Beamsville, Jordan, and Niagara-on-the-Lake. The latter connects with Niagara Falls via a scenic floral parkway. Niagara Falls and Niagara-on-the-Lake can get very overcrowded in tourist season (June-September), so arrive early if you want a parking spot. The town of Niagara Falls also has a casino.
If you want to nip into the USA, cross over the Peace Bridge and you are in Buffalo. It is only a 90-minute ride from Toronto. The Albright-Knox Museum – on upper Elmwood near the University of Buffalo and in an area of stately mansions – is worth a visit. Be sure to bring your passport as valid ID, and if you are not a Canadian or American citizen, check beforehand about a day visit. There are fun places to browse and eat south of the museum on Elmwood.