You can take your bike on the GO Train. Free of charge. This allows you to get to UTSC from any GO Train station on the Lakeshore East Line. There are some restrictions, though. For example, you can't take your bike into Union Station during rush hour. Check the GO website for the specifics. In short: if the train is packed, you can't take your bike on the train unless it's a folding bike. For my route, coming from Danforth GO Station, this is not an issue. In fact, sometimes I'm the only person in the train car. Trains run very frequently, especially during rush hour, every 15-30 minutes.
Exit the GO Train at Guildwood Station. From there, cross Kingston Road and say goodbye to cars. You won't see another one for the remainder of your commute. Next, you'll pass the lovely Greenvale Park and enter the Highland Creek Trail. It's a paved recreational trail, ideal for cycling. However, keep in mind that there is no winter maintenance. Every other season, however, is a treat! You'll see a lot of flowers and wildlife, including deer, rabbits, coyote, foxes, owls, and all kinds of other birds. While on the trail, just follow Highland Creek. It's all downhill, which makes the cycling even more enjoyable.
Once you've passed Morningside Park, you'll enter UTSC grounds. Cross the metal bridge and then turn left, slightly uphill. The last few meters are relatively steep, so you may want to walk. You can either lock your bike somewhere in the valley or take it to the upper campus. I usually take it to campus. Note that if you have an office and it's large enough, just take your bike inside. And in case you need to fix something on your bike, UTSC has a bike repair station, located between the AA building and the Student Centre.
Once it's time to head home, go back down the valley from the main UTSC campus. Continue downhill on the trail that follows Highland Creek and pss the UTSC playing fields and tennis courts where parts of the Pan Am Games were held. Cross Old Kingston Road and pass through Colonel Danforth Park, then the Lower Highland Creek Park. As you go further downstream, the vegetation changes quite a bit, you'll see a lot of ferns. And keep an eye open for Trilliums in the spring!
After you pass under the GO rail tracks, you will reach Lake Ontario. This sudden change of scenery from a ravine and dense forest to the vast openness of Lake Ontario is quite impressive. If you're lucky, you might even catch a rainbow. When you're done taking pictures of the rainbow, cross the metal bridges and turn left, following the Waterfront Trail. Winds in Toronto are typically coming from the west, so you'll have a pleasant tail wind on most days.
After 5 or 10 minutes, you are in Port Union Waterfront Park, and the Rouge Hill GO Station appears on the left. You can enter it directly from the trail. Trains run every 15-30 minutes on weekdays so you don't really need to worry too much about missing your train. Just take the next one. And if you're lucky, you might catch an express train. I often take the 17:43 train from Rouge Hill, which takes me back to Danforth GO Station in 12 minutes.