The Martin Goodman Trail traverses the length of Toronto along the Lake, from Etobicoke all the way to the Beach in eastern Toronto (Woodbine Avenue area). At least that's the theory. If you're on foot, you'll find that the trail is really divided into two parts, east and west. In the area immediately south of the downtown core ( from Bathurst to lower Jarvis) the Trail disappears completely, replaced by sidewalks and a bicycle path. This area has the Lake on the south side, but busy traffic on the north, and aside from a few ducks and the water, not much wildlife or nature to admire. So, if you're looking for some nature, you're best off getting to the trail where it starts in earnest ( see map below).
Martin Goodman Trail -West
This part of the trail begins at Bathurst, south of Lakeshore Boulevard, at the corner of Quuen's Quay. The trail is paved, and is shared with bicycles, so stay alert. The trail heads west, following the lakeshore for the most part, although there are some exceptions. As a pedestrian, you'll find that there is a sidewalk that consistently follows the water, while the path diverges from the Lake at times. The most scenic option is to follow the sidewalk, which will keep you close to the water. You will be led into Ontario Place, a former summer fairground, where numerous boats moor in the summer. (This 2km from the start). After Ontario Place, the sidewalk and the trail meet up again and carry on west, leading to the Sunnyside Beach, and a few artificial peninsulas at Humber Bay, which have plenty of wildlife, especially birds.
This walk westward is a great way to see Lake Ontario, and will take you through numerous parks, with tall old trees. In winter, there are lots of different kinds of ducks around, some of which, like Oldsquaws, fly down from the Arctic. During the summer, you're more likely to see mallards, cormorants, swans, thousands of Canadian geese, and herons. Even near Ontario Place, which is quite busy, it is not uncommon to see night herons, which are short blue birds, perching on docks, waiting for a fish to come by. There are also lots of swallows, sparrows and bats at night.
After you get through Ontario Place, the sidewalk and path rejoin to wander along the Lake. Map-wise, you're now getting near the bottom of Jameson Avenue( 4km since your start) . There's a minor hill to go down, then you enter an area where the paved trail heads straight ahead, while you have the option to walk along a lovely boardwalk that hugs the Lake and a series of beaches. Take the boardwalk. (This choice happens where you see a pedestrian bridge that has crossed the highway and whose staircase joins the trail).
The boardwalk is lovely, and is supposed to be only for pedestrians, so bikes and rollerbladers stick to the paved trail (mostly). You'll walk past beaches, a kid's playground, a public pool, and then to the Sunnyside Pavillion and beach (5.5 km since your start). This area is very scenic, but not wild. But at sundown, it is gorgeous.
July and August are the only months when the water is warm enough for swimming ( trust me, I have tried earlier and later). The beaches have lifeguards on the days that the water is safe to swim in. The water is safe most days, except after rainfalls. The beaches have numbers you can call to check on whether it is safe or not. You can also check online here. The beaches themselves are sandy, and the water is usually okay in the summer, although it can be a bit murky in some spots. If so, just walk down shore a bit and find a clearer patch ( the issue is algae, not pollution).
As you walk past Sunnyside, with its cafes and beach volleyball tournaments on summer nights, you'll reach a long stretch of paved trail that will lead you to the Humber River (8 km since your start). It is hard to miss, since there is a large white pedestrian bridge that stretches across it. This is the official end of the trail, although it continues past here into Etobicoke, reaching Humber Bay Park, a lovely place to visit, which I talk about here.