One of the best day trips for a lovely afternoon in Toronto is to take the ferry over to Toronto Island for a day—while the weather is at its most pleasant during the summer, the ferry is in operation your round.
This place is amazing, with its numerous parks, bike paths, and amusement park. However, word to the wise, please bring all the necessary food with you as the options on the island can be pricey! (The ferry is quite reasonably priced, but make sure you have plenty of water and food for your adventures outdoors).
Have some fun there, relaxing in nature with great views of the CBD of Toronto.